I've long held a theory that in the main Jinyong-verse, when people reach a certain level of martial arts skill, they face a problem of massive diminishing returns. In RPG terms, this would be where you reach the level 'softcap' of the game - you continue to gain XP, but the next level require so much XP as to be basically unobtainable, or where your damage/hp caps out at 999/9999, etc. In the Jinyong-verse, I believe this softcap is at the Greats level.
This would explain a large number of inconsistencies in growth patterns we see in Jinyong, and how everything seems to cluster at roughly that level. For example - in DGSD, Murong Bo was initially so completely outclassed by Xiao Yuanshan that he was utterly terrified of the man, but after the two spent similar amounts of time hiding in Shaolin, they somehow reached a level of parity. Jiumozhi was awed by and in admiration of Murong Bo when they first met (at a time when Murong Bo was inferior to Xiao Yuanshan), but by the time of the events of DGSD he too had reached a level of parity to both of them.
In LOCH and ROCH, we see all four Greats advance roughly by the same amount, and Jinyong also seems determined to keep Wang Chongyang roughly on par with them despite the passage of decades. Even Qiu Qianren somehow keeps up with everyone! Similarly, Guo Jing quickly skyrockets in LOCH to near-Great status, but even decades later in ROCH he only remains roughly on par/slightly better than them at best. Yang Guo eventually finds that snake gallbladders no longer boost his energy that much in ROCH, while in HSDS Zhang Wuji uses 9Yang/QKDNY to reach a level that's basically comparable to the century-old ZSF, who in turn comments that he's probably on par with GJ/YG.
All of the above just makes no sense, unless there is a 'softcap' on your martial arts 'level' that everyone essentially reaches. Some reach this level earlier (Zhang Wuji/Xiao Feng), some reach the level later (Zhang Sanfeng/Murong Bo), but once you reach it - you are basically on par with everyone else.
BUT. There are a few techniques that seem to allow you to break through this softcap. As in any good RPG, I call these 'limit breakers' - but in the Chinese context, it almost seems like they are more akin to breaking through the 'mortal' cultivation level and direct you into the 'Immortal' level.
Below are my list of martial arts cultivation 'limit breaker' techniques, and the 'area' they limit break.
Xiaoyao Sect from DGSD - Internal Energy Limit Breakers
Tianshan Tonglao's 'Never-Aging Eternal Youth' technique and Wu Yazi's 'Beiming Shengong' technique.
Xiaoyao Sect is the closest thing in Jinyong to having something akin to a Daoist 'Immortal' sect, and as you expect, it has the most limit-breakers. All of Xiaoyao Sect's practitioners gain the benefits of tremendous longevity; when Wu Yazi transferred his internal energy to Xu Zhu, he aged so much that Xu Zhu thought that he had passed out for 50 years! In turn, even though Wu Yazi was probably 80+ years old, Tianshan Tonglao was completely shocked at him dying, stating "how could he die, given his incredible internal energy?!"
Now, Tianshan Tonglao's 'Never-Aging Eternal Youth' technique is even more advanced in this regard - it appears to essentially allow for eternal youth and maybe even eternal life (the reversion to a child effect was due to fire deviation caused long ago by Li Qiushui), allowing one to stay in maximum physical condition while continuing to accumulate utterly dominating levels of internal energy. If this isn't a limit breaker, I don't know what is.
Similarly, Wu Yazi's Beiming Shengong is utterly ridiculous; it allows you to perfectly absorb other people's internal energy, blowing past any limitations of natural growth. What we see from Xu Zhu, when he basically took in the internal energy of all three Xiaoyao elders to become arguably the greatest internal energy powerhouse of the Jinyong-verse, is testament to how broken Beiming Shengong can be.
Li Qiushui was a bit weaker than both of the other two, and it shows - while her Xiaowu Xianggong is amazing in allowing the user to emulate any type of martial arts technique, in and of itself it isn't a limit-breaker - which is why she was never a match in a face-on clash against either TSTL or WYZ.
Sunflower Manual from XAJH - Speed Limit Breaker
I think everyone knows what 'limit break' the Sunflower Manual applies - pure speed. While there's almost certainly an internal power component to it as well, the Sunflower Manual is primarily a speed limit breaker that allows its users to reach such ridiculous heights of speed that no other martial artists can compete in this arena. More importantly, that's in its bastardized form! The original Sunflower Manual was kept by Shaolin - two incomplete copies were stolen by Huashan Sect (later taken by the Sun Moon Sect and learned by DFBB), and then Lin Yuantu combined them into his own version, this 72-stroke Pixie sword technique. This means we have never seen the full might of the true Sunflower Manual, and I shudder to think of how powerful it truly is.
In addition, while we react with aversion to the castration requirements of the bastardized version, keep in mind that in classical Taoism, one of the ways to achieve Immortality is by refraining from sexual intercourse - hence we have Zhang Sanfeng, the hundred-year-old-virgin. So in a way, the bastardized versions of the Sunflower Manual that we see are kind of keeping in touch with some of the tenets of Immortal cultivation!
Dragon-Elephant Prajna from ROCH - Strength Limit Breaker
I know this will probably shock a lot of people, but I firmly believe that the Dragon-Elephant Prajna technique which Jinlun Fawang uses is a limit breaker, a strength limit breaker. It literally doubles in strength with each level, and has a maximum theoretical level of 13. Jinlun Fawang only reached level 10, but was already at a state where he had essentially surpassed the Greats in power! Not even Zhou Botong, arguably the strongest Great by the end of the Condor duology, was willing to meet him blow-for-blow, and instead used Vacuum Fists to ablate the power of his blows. This means in theory, someone who can master Dragon-Elephant Prajna to the fullest level would have eight times the power of Jinlun Fawang. This definitely qualifies as having blown past the power limits!
The ties to 'Immortality' are a bit more tenuous here, but the dragon is the most divine and powerful creature in Daoist mythology, so there's at least some connection as well.
Dugu Nine Swords from XAJH - Technical Limit Breaker
Dugu Qiubai is the closest thing we have to a Sword Immortal in the Jinyong-verse, although he self-styled as the 'Sword Demon', and this technique of his is so bizarre and hacked that it must be included. I believe that the Dugu Nine Swords are a technical limit breaker, on the level of being reality breaking. After spending just ten days studying this technique with Feng Qingyang (and no internal energy boosters!), Linghu Chong reached a level of swordsmanship that surpassed almost everyone in the martial world, and was able to pull off feats that just make no sense, such as blinding 15 elite masters instantly when he had virtually no internal energy. In addition, DG9S has its trademark ability of 'becoming stronger when you face stronger opponents' - but more importantly, you keep that level of strength after finishing the fight! With each powerful opponent Linghu Chong faced, his sword techniques continued to skyrocket. @Ken you know how the Hulk gets stronger as he gets madder? Now imagine if Hulk calms down but still keeps his 'mad strength' as his new 'base strength' level. I shudder to think how powerful LHC would have become if he was actually ambitious and motivated to train hard, or if Feng Qingyang spent more than just ten days teaching him. Considering the nigh-reality breaking powers of DG9S, I think it definitely qualifies as a technical limit breaker.
Taixuanjing/Heroes Island Technique from Ode to Gallantry - All-Round Limit Breaker
This is one of the most mysterious techniques in all of the Jinyong-verse. It's never explained in detail and only appears in the final chapters of Ode to Gallantry/Xiakexing, but it propels Shi Potian to a prajna-state where he is able to defeat both of the Island Lords at the same time and essentially exhaust them to death - and the Island Lords are basically at Greats-level imo, leagues above everyone else in the world of Ode to Gallantry! There is also an Immortal connection here as well - the technique (and the name of the book) are based off of Li Bai's poem, and Li Bai is popularly known as both the 'Immortal of Poetry' and a 'Sword Immortal' in legends and myths. All this combined convinces me that the Taixuanjing technique is a true limit breaker, possibly one of the most profound techniques in the Jinyong-verse, which points the users towards a path of transcending the mortal limits.
Honorable Mentions:
Heavy Iron Sword + Heavy Iron Sword Technique - This is disqualified because there's a weapon component/requirement to it, but it would almost certainly qualify as a limit breaker otherwise. Yang Guo's usage of the HIS and the HIS technique allows us to see something which is almost never seen anywhere in the Jinyong-verse - a clean and fairly quick defeat of a Greats-level fighter in Qiu Qianren. Greats-level fighters are never defeated like that, with the exceptions of Shi Potian and Sweeper Monk. And this was Yang Guo as someone in his late teens! Even later on, when Yang Guo had advanced by leaps and bounds, he regretted not bringing the HIS to his fight with Jinlun Fawang - which strongly implies that he'd still be considerably better using that sword and that technique - and he was already beating Jinlun Fawang with his 'regular' Sad Palms! I consider this technique a 'force limit breaker', but because it has special material requirements, ultimately it did not make the cut.
Zhenwu Formation - This ultimate formation of Zhang Sanfeng deserves an honorable mention. Although it is a multi-person formation and thus doesn't qualify, it is still a limit-breaker in a sense because it allows 7 people to unleash the power of 64 first-class fighters with each blow! This is definitely something that is limit-breaking, but again, it does require seven specific people using a specific formation/internal energy, so it doesn't quite make the cut. But, it's still definitely badass. Alas, Zhang Sanfeng gave up on his attempt to create a formation that one person could use to achieve the same effect - otherwise, it might very well become the crowning limit breaker of the Jinyong-verse.
9 Yin/9 Yang - Surprisingly, these two coveted techniques are not limit-breakers. Despite how much they were hyped up and their many benefits, they just don't break the limits. The presence or absence of 9Yin didn't seem to make a difference in the power level of the Greats - even Yideng with both YYZ and 9Yin was said to be just the tiniest fraction better than Qiu Qianren (who never learned 9 Yin), while 9 Yang quickly propelled Zhang Wuji to Greats levels but then no longer seemed to further any improvements. These are great techniques - but not limit breakers.
Tendon Altering Sutra - This technique is also not a limit breaker. It appeared in two places - DGSD with You Tanzhi, and XAJH with Fang Zheng. In the former, even combined with the Ice Silkworm it only propelled YTZ to a level of internal energy that was essentially on par with Greats like Xiao Feng, while in the latter the 'pure' version only allowed Fang Zheng to reach a level that was somewhat superior to Ren Woxing. Therefore, while the Tendon Altering Sutra is powerful, it is not a limit breaker. Quite frankly, there's nothing in Shaolin's arsenal that seems to be limit-breaking, which is another reason why I've always had a suspicion/head-canon that Sweeper Monk is actually related to Xiaoyao Sect somehow - but that's a whole 'nuther topic.
Shen Zhao Jing - This ultimate technique from A Deadly Secret is pretty impressive, but overall it just seems to be on par with 9 Yin/9 Yang. I'm not as familiar with ADS as I should be, but I *think* there might be an argument for it to be a limit breaker in terms of healing ability, as its reputed to be able to bring the dead back to life, but that's pretty much it.
Book & Sword, SSWRB, Flying Fox, Deer and the Cauldron, etc. do not have even normal 'Greats' level techniques, much less limit breakers. By curious coincidence, all of them are set in the Qing dynasty years.
And there you have it! My 'softcap' theory and the limit breakers that can theoretically allow one to overcome that 'softcap'. Can you think of anything I have missed?