Seems like Valve disagrees. They have had excellent results porting Steam and L4D2 to Linux. As a matter of fact there was an article a couple of days ago where they have stated that the Linux version runs faster than the Windows version. This is on the same hardware as well using OpenGL as opposed to DirectX. Now with that said when running it on OpenGL under Windows as opposed to DirectX it ran faster as well when they applied the knowledge and tweaks they gained from Linux but in either case it still runs fastest on Linux. Gabe Newell also blurted that he would like to see all of the Steam games available on Linux not just a select few.