Exactly, now get on with it. Borg couldn't make the transition all the way against the best players and neither could other defensive clay courters. The only players who were successful at Roland Garros first and then in New York were ones that were offensive (Lendl/Agassi/etc). The only exception = Wilander who only won one time which was on his best GS year but aside from that, he didn't really have a fantastic record at the US Open although he made a final the previous year and got bageled by Lendl
Nadal can really only hope to play great matches defensively when it matters most and hope that he doesn't run into anyone red hot because seriously, he isn't going to play like a Soderling/Del Potro now.
but Wilander won Cincy four times, so it's not as if he was a mug on fast HC
Rafa's movement in HC isn't as good as his movement in natural surfaces and it has to do with two things: pain/fear of injury, he knows HC can totally kill his knees if he's not careful and it doesn't come naturally for him, not the way clay does
another problem is that he can't flatten his shots well enough and his topspin gives his rivals a lot of time to do their own strokes, his forehand in a low bouncing surface screams (specially for tall guys) smash me
Rafa's serve has improved a lot, but he's not someone who will win easy points with it all the time like natural HC players do, he needs work in his kick serve and has to learn when to risk a first serve, he plays too safely and you gotta be aggressive in HC