I have use SL under both Windows and Mac OS X on my late 2009 27" iMac and not noticed any grave performance differences (even though I haven't been looking out for them either)...
As for the OpenGL implementation of Apple: I must leave it to Apple, that while the performance cannot keep up with the implementations by ATI and nVidia, the stability has been so far outstanding. And they are making giant leaps forward; the last version, Snow Leopard, supported OpenGL 2.1; that's not even par with DirectX 9. The current version supports OpenGL 3.0, which is not quite the current state of the art, but it's a solid base to work from, comparable with DirectX 9. The upcoming version, Mountain Lion, supports OpenGL 4.1. That's a massive improvement in very short time. Of course the drivers do not have as much performance optimizations as nVidia's and ATI's drivers, but they are a lot more stable. nVidia and ATI both support OpenGL 4.5 afaik, and that for more than a year.
So, while I originally was very disappointed with Apples policy to create their own drivers from scratch instead of trying to work with the manufacturers to adopt existing drivers by the manufacturers. The consequence was naturally that the drivers in Mac OS X cannot only not keep up with Windows, but also not with Linux, which does use highly optimized state of the art OpenGL 4.5 drivers as well. However, Apple did provide a homogenous platform to all software developers and a stable environment to its users, which seems to have worked out despite all its shortcomings compared to the competition. And now that Apple is catching up, it looks like their somewhat headstrong path to more independance from the manufacturers and more immediate control of their graphics stack seems to slowly work out. Apple is not on the level of Windows and Linux, but it's getting there. With a system under their own immediate control.
I am just curious how Apple will handle those intel Chips. I don't have a Sandy Bridge Mac, but afaik,the on Chips intel HD 3000 have been ignored so far. Giving the massive scaling up on the new Ivy Bridge generation, the HD 4000 chips, that would be quite a waste. Especially the cheaper MacBook Pro and iMac models wouldn't really need an own graphics card anymore; if those chips were properly supported.