A few thoughts for discussion.
First up, Linghu Chong.
It seems that his internal energy was still below that of Zuo Lengchan at the end of novel. He was able to slaughter Zuo with better technique, but his hand trembled when he clashed swords with Zuo, and after a while Zuo's strong force made Linghu Chong's weapon fly out of his hand. This implies that Linghu Chong couldn't match Zuo Lengchan in the field of internal energy.
Xiao Feng.
By age 30, he had reached Great-level. Starting age 7, he was taught Shaolin martial arts by Reverend Xuanku, one of Shaolin's best. At age 16, he became student of Beggar's Union chief Wang Jiantong. Given these information, one may presume that teenage Qiao Feng was already a powerhouse. However, there is some evidence suggesting this was the case at all. In fact, evidence hints that Qiao Feng at age 16 may have been BELOW the level of 16 year old Yang Guo (who was weaker than Huodu).
The evidence is that when Qiao Feng was around 16 (but not before then, since the novel said he didn't meet Chief Wang until age 16), he learned martial arts from Elder Xi of Beggar's Union. The exact description from the text is 提到昔年自己指点他武功. ...When it was brought up that I [Elder Xi] provided martial arts instruction to him [Qiao Feng] years ago... This implies that Elder Xi would have been more competent in martial arts than 16 year old Qiao Feng. And yet, some 12+ years later, Elder Xi was barely a match for Evil #4 Yun Zhonghe. For all we know, Yun Zhonghe is at best 10% of present-day Xiao Feng's level. So from age 16 to 31, when Xiao Feng doubled his age, he would have increased his kung fu mastery by about 10 fold.
Based on this information, I don't think 16 year old Qiao Feng (below Elder Xi's level) could launch a deadly LDA to kill a second class fighter like 15 year old Murong Bo demonstrated. So, 16 year old Murong Bo should > 16 year old Qiao Feng. And yet, 60 year old Murong Bo = 30 year old Qiao Feng. If my assertions are correct, it would mean that Qiao Feng's rate of improvement is actually frighteningly fast, which helps explain why he noticed an improvement within half a year of NOT touching his training at all.