The only reason the linux drivers for opengl and the sorts run faster is because valve pushed the developers at amd, nvidia, intel, etc to make them better so the source engine could run better on there. They may have made l4d run faster (315 fps on a test), but don't forget the sentence that followed right after:
Interestingly, in the process of working with hardware vendors we also sped up the OpenGL implementation on Windows. Left 4 Dead 2 is now running at 303.4 FPS with that configuration.
303 isnt 315 but its certainly much better than 270, and at the time the windows drivers weren't even supposed to be worked on- so perhaps if nvidia/amd/etc focused purely on windows with the same problems they encountered with the linux drivers, it's drivers would be up to par with linux's, too. The only reason I can think of that makes shaders run better on linux is because of the compilers are different, but those are built in with the drivers, so its not the fault of the OS which determines what runs faster- its the driver developers.
Someone should run blockland on windows and linux and brown townyze the generated shader assembly to see how different it is.
Poll