There's this strange situation with OpenGL drivers ATM, especially on Linux. Strictly speaking, latest NVidia driver itself is slightly faster in OpenGL/Linux, than in Windows/D3D. However, seems that shader performance when running with it is much lower. This is the 50% speed difference people are talking about. Shader performance is largely a factor of shader compiler embedded in the driver. NVidia is working on this, and if they make it as fast as on d3d, cards like yours will probably benefit the most, due to the aforementioned bandwidth issue.