When I ran Linux, every time I updated my nVidia driver and did performance measurements, they seemed better, sometimes by a surprisingly large amount. I never did measurements with other systems. But if I were a graphics artist, the first thing I'd do with an optimized driver and engine is increase my polygon budget until I started to stress the engine/drivers again. At least, that's what I do with my own 3D programs... :)
I have to wonder how much that's got to do with unoptimised drivers and unoptimised engines. It annoys me that there are some games I'll never be able to run (on PC) as well as they ran on the devs' PCs.