Well I just did a simple test of my own, very simple actually. All I did was make an nCloth object react with a ball and move the ball during playback. Under Windows, the frame rate was descent and you could move the ball around and see it interact. It wasn't perfectly smooth, but it was alright. In OS X, it was MUCH worse. Noticeable difference in frame rate. Trying to move the ball during playback was extremely delayed.
So it appears using the same GPU yields better performance in Maya in my test. I don't know if there will be a difference in performance depending on what you are doing, for example dynamics versus straight up high poly count. But in my test, dynamics were much smoother under Windows with the same GPU.
mBox, if you get a chance to make your own test scene, let me know what you find. But, at least for dynamics, it is pretty obvious that Maya runs better in Windows 7.
EDIT: Ok, I just did a quick test with a fully textured model I made a few months back. It's a high res model with fairly high res maps. And just rotating around it was noticeably fast in OS X. Can anyone explain why dynamics are much more responsive under Windows when rotating around a high res model is much faster in OS X?
And just to be sure, if I were to get a Quadro card, almost any Quadro card, it will be much faster under Windows than my 5870 in OS X because of CUDA support, correct?