Ehh... any proofs to back up what you claimed? I have proofs for the other way around.
Anyway, I'm not comparing XP vs Vista. Performance-wise, Vista DX9 loses to XP DX9 already... and that's not a driver issue. I mean... you can argue that DirectX 10 drivers haven't matured yet, but what are you going to say about DirectX 9 drivers? That's one thing. I can also run Windows Vista with a Celeron processor (single core), 1 gig of RAM, and with an Intel graphics processor, too. And of course it's not going to degrade your performance so drastically it's noticeable. And I'm not bashing anything here, just stating the fact that Windows Vista degrades performance to the point where the gain from DirectX 10 is not even comparable to DirectX 9 under Windows XP.