The truth is Games will run better on Windows, one of the reason's is because most developers choose to develop for Windows, and I don't blame them, as Apple has not really done much in terms of making their computers capable in the graphics department..
Also sad thing is Mac OS doesn't even have support for the latest OpenGL 4.1, we haven't even seen it on Mountain Lion..
There are very few REAL Mac games out there, most of them are cider ports, so generally you find the performance to be quite poor on the Mac side, not only does this mean you need a more powerful GPU to run Mac ports, but they still perform significantly better on Windows..
Even with native mac games I find the performance to be lacking slightly when compared to Windows version, maybe its because there is no DirectX in the picture..