Phoronix reports Valve's Left 4 Dead 2 runs faster with the Source OpenGL rendered in Linux than on Windows 7 with DirectX! On a Core i7 3930K system with GeForce GTX 680 the game ran at 270 FPS on Windows 7 SP1, while the OpenGL renderer on Linux managed to achieve 315 FPS.
The Linux driver optimizations come via collaborating with Intel, AMD, and NVIDIA on their respective drivers. Valve has long communicated with AMD and NVIDIA on their binary Linux graphics drivers and then in recent months engaged with the open-source Intel team -- see Valve & Intel Work On Open-Source GPU Drivers.
Using a NVIDIA GeForce GTX 680 graphics card with an Intel Core i7 3930K processor, Windows 7 SP1 was running Left 4 Dead 2 with the Direct3D renderer at 270 FPS while under Linux with OpenGL they are now at 315 FPS! Using the OpenGL renderer on Windows isn't also quite as good with its average frame-rate at around 303 FPS.