It's more a matter of OpenGL vs DirectX than drivers/OS.
Generally, DirectX versions of the game are optimized better since developers rather invest their time improving the most played version of the game (Windows).
If Steam pushes Linux for gaming, developers may take OpenGL more seriously. Valve worked on a OpenGL port of L4D2 that they tried to optimize as much as possible just to prove it's possible to get better performance than with DirectX.
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However there is no real incentive for developers to optimize their OpenGL ports that much right now, hence the performance discrepancy. DirectX is a proprietary API made by Microsoft so it will stay Windows-only.