Phoronix published a review on the AMD Catalyst Linux OpenGL Driver Now Faster Than Windows Driver In Some Tests A quote from the article:

Earlier this week I showed benchmarks of AMD's incredible year for their open-source Linux driver and how the open-source Radeon Gallium3D driver moved closer to performance parity with Catalyst. One of the lingering questions though is how does the Catalyst 14.12 Omega Linux driver from December compare to the latest Catalyst Windows driver? Here's some benchmarks looking at the latest open and closed-source drivers on Linux compared to the latest Catalyst Windows release.

 AMD Catalyst Linux OpenGL Driver Now Faster Than Windows Driver In Some Tests @ Phoronix