This is a very valid argument, though I have a more simplistic view in the comparison of martial arts.
1) I think DGSD characters do not "appear" to be stronger, or more forceful. They actually are. This fits into the central theme of DGSD, of irony based on the fall of ridiculously strong characters (XYP elders, XF) the emergence of ridiculously strong characters who were previously weaklings (DY, XZ), and how the rise and fall of characters are tied together in a series of stories which highlights the complexities of life. There is a need for them to display powerful martial arts, but I don't think "powerful" and "refined" are binary and necessarily dichotomous; they are only written explicitly to be powerful as you had pointed out, to fulfil the needs of the story, but there is nothing ostensible about this.
My personal belief is that DGSD features the most elite martial artists.
2) This brings us to a very natural question of, ok it's fine and all for people to make baseless claims of "I think, I think..", but come on, where's your proof? Point out specific passages and feats of superiority then, if you are so sure.
My answer is that this isn't about LDA and SOTTAWPC, but what is behind JY's writing the stories? What are his thoughts and intent?
The intent must surely and irrevocably be to sell stories, and if strong martial arts are needed (because the Greats are by definition, Greats) then these bits will be written. Certainly I dont think, JY would consider, I can't write Guo Jing too powerful, because XF's XL18Z is the best. He isn't interested in the comparison, and he writes GJ in the way to fulfill the story's needs, even if somewhere down the line GJ might ostensibly be more powerful than XF. He cant possibly hold back on his description thinking, "oh no if i write that GJ does this and this then he might appear better than XF (or the other way around since DGSD came later)".
Another common point is that refinement is displayed for example, through HYS, who invented and developed his own set of Martial Arts, as compared to DGSD where people just learnt existing martial arts. Again I feel this is somewhat unfair. THe martial arts in DGSD was already very high, why do I need to learn new martial arts when with level 9 XWXG I can already trash Shaolin and imitate every skill? More importantly JY had deliberately set the context of 4 greats, neither of whom are the main protagonist. He can't have overlapping martial arts between the 4. So naturally he needs ot differentiate between the martial artists.
In any case, how many martial arts can an author come up with (or want to come up with?) I think YJJ and XL18Z are very good examples of hey this works well, let's keep it. Or an even better example, DGQB and his set of martial arts.
So in the end, I think author's intent is a far more important point than the comparison of feats.