For the Xonotic game with high quality visuals and a 2560 x 1600 resolution, the Windows 7 Catalyst driver continued to be faster than Ubuntu 12.04 with the same month's Catalyst. For this test the Windows 7 stack was 14% faster than the popular desktop Linux distribution. The NVIDIA driver meanwhile did better under Linux compared to the Windows build.
With Ultra quality visuals for Xonotic, Windows 7 with Catalyst on the Radeon HD 7950 was 7% faster the Ubuntu 12.04 LTS software stack.
Lastly, with the "ultimate" visuals, the results were much closer between the two competing operating systems with the frame-rate difference coming in at less than 4%.
While for the most part the Catalyst driver performance for the Radeon HD 7950 graphics card was close between Windows 7 and Ubuntu 12.04 LTS, there were some OpenGL performance differences. The performance should largely be the same since the Catalyst code-base is mostly shared across AMD's supported operating systems with the driver weighing in at millions of lines of code. However, there were some differences, including when the NVIDIA Linux driver would be faster than its Windows counterpart, the AMD Catalyst Linux driver would struggle in comparison. At the end of the day though, the differences are not likely to make or break your decision in choosing between Windows and Linux when using the company's proprietary graphics drivers.