I believe a ROCH Great actually can toy around 3 LOCH GJ's. I will provide my explanation...but I admit, there are some odd inconsistencies in JY's power scale that also makes me raise an eyebrow sometime.
let's assume LOCH GJ is the standard 70% of a Great.
By ROCH GJ has at least surpassed LOCH Greats because after GJ and OYF's initial clash we have the below:
"Guo Jing meditated on the boat, and after a few days, his energy has recovered by more than half. Huang Rong and Guo Jing raised the topic of Ouyang Feng, saying they hadn’t seen him in ten years. Not only does it seem he hasn’t aged much but his kung fu has improved."
There is no indication of how much improvement, but I would think that the gain has to be significant for GJ and HR to bring up OYF's newfound prowess.
Then when H7G fights OYF we have this excerpt:
"Since the second Mount Hua competition, Hong Qigong had not seen Ouyang Feng for over ten years. Although Ouyang Feng’s mind was unclear, he practiced the Contrary Nine Yin Manual”; the more he practiced the stranger his kung fu became, and the stranger it became the more powerful he became.
Guo Jing and Huang Rong had recited a small portion of the manual to Hong Qigong; it made an impression on his kung fu and great progress in his martial arts. The final stage of the “Nine Yin Manual” is superior to the “Contrary Nine Yin Manual”, although Hong Qigong only knew a little; he wasn’t inferior to Ouyang Feng"
This indicates that the author clearly indicates reverse 9-Yin made OYF strange, yet the strangeness has contributed to his power. Yet with only the final stage of 9-Yin, H7G would not be inferior. This indicates that both OYF has gotten stronger and that 9-Yin really is "as good as advertised" to give such a boost to H7G. And tangentially, it must've done great wonders for GJ, which is now stronger than a previous LOCH Great
Then moving on, we discover there is GWM who is as strong as the aforementioned GJ. When GWM and the Mongol mercs attack QZ palace we have this quote:
"The Imperial Priest and the other Mongolian warriors had higher martial arts than the Five Masters of Quanzhen. It was practically impossible for the Quanzhen School to even produce someone like them now."
We know that the QZ elders have improved in martial arts and QCJ is the strongest, yet the 5 QZ elders are on average weaker than the Mongol mercs. And this is coming from the QZ elders who just came out of meditation after creating a new formation. Why I am bringing this up is to establish a baseline. Because LOCH QCJ was around par to MCF. And if these Mongol Mercs are at least as strong if not stronger than an ROCH QCJ, then they are stronger than everyone in LOCH wulin except maybe end LOCH GJ
Now we fast forward to post 16 years and we have this:
"Xiaoxiang Zi was still angry about the fact that Yang Guo broke his arm at Mount Zhongnan sixteen years ago. Although his internal strength had increased a lot, he knew he was no match for Yang Guo. He did not bother to look at Yang Guo again and turned around and started to descend the stairs."
This tells us a few things. As I established before, Mongol Mercs are quite strong and in these 16 years XiaoXiang Zi's internal was still increasing! And it didn't only increase slightly, it improved a lot-- which no one knows exactly by how much, but shouldn't be small gains if the author wants to describe the increase in this manor. Yet we know what happens right after when YG and HYS basically make them ping pong balls. Post 16 year Greats are actually extremely strong...especially if we want to factor in Elephant Pranja doubling GWM's internal
Summary:
1) There is definitely improvement from LOCH Great to end ROCH Great in terms of martial development
2) That development shouldn't just be tiny or only fractional.
3) Mongol Mercs by end of ROCH should be stronger than everyone in LOCH other than the Greats or GJ.
4) ROCH Greats basically play around with Mongo Mercs for fun
In comparison to SPW:
If we assume (again, assuming) that RWX/FZ is an LOCH GJ, then it means that LHC and XWT are only around Mongol merc levels or lower. Which means an ROCH Great can play ping pong with these guys and basically one hit KO them.
However, because I think like many old posters before, that RWX is an LOCH Great lvl. And that LHC and XWT should be somewhere between Mongol Merc and LOCH GJ. Thus, why DFBB (who is around ROCH Great) can toy with them, but can't quite ping pong them yet.
It's getting late.
I have some follow ups to your other points
And I discovered some interesting examples which demonstrate how internal and external technique are not always interdependent. And excelling at one can accomplish certain feats, yet isn't sufficient to increase overall fighting prowess. It also explains why GWM could've gotten double internal post 16 years and yet isn't leagues above the other Greats