The RVD example above is an interesting one for this purpose. RVD's "improvement" was, at least initially, really just a product of not working in an environment where his match could last as long as he wanted, and working with workers who forced him to keep moving. But the quality jump was huge - ECW RVD in 2000 is almost impossible to watch at times, because the matches are so slow and formulaic, but WWE RVD in 2001 is a joy to watch, having lots of fun matches up and down the card. But if left to his own devices - wouldn't he have wrestled the same way in 2001 that he did in 2000? (I'll say - I haven't watched enough of his post-2001 work recently to comment on how he changed later on).
And this would go to the Danielson point - if you believe he'd wrestle a completely different style, basically his 2006 style, in the indies now (not sure that's true, but take it for the sake of argument), then he hasn't improved, he's just adaptable.