It's natively (more or less) running now, only on apple's ("Superior" - as they call it) hardware. I don't see why the performance would be much different than on a PC, except for hardware driver stuff.
DirectX is the more common and obviously efficient api for game developers. That makes most of the current games be more optimized on DirectX rather than OpenGl. I can imagine a very noticeable performance boost with windows (directX).