With the imminent release of Mesa 9.2, out this morning are Radeon "R600" Gallium3D driver benchmarks comparing Mesa Git master (9.3-devel) to Mesa 9.2 Git, Mesa 9.1.6, Mesa 9.0.3, Mesa 8.0.5, and Mesa 7.11.2. These are the past two years worth of Mesa releases and testing occurred on an ATI Radeon HD 4890 (RV790XT) graphics card.
The test platform used throughout testing was an Intel Core i7 4770K Haswell system that was loaded with Xubuntu 13.10 and was running the Linux 3.11 kernel with Radeon DPM support enabled. Swap buffers wait was of course disabled as well for the xf86-video-ati DDX. The Mesa 7.11/8.0/9.0/9.1/9.2/master branches of Mesa's Git repository were built as of 26 August using the latest code in each branch at the time of testing. Git master was used just for seeing if any additional OpenGL performance optimizations have squeezed in since the Mesa 9.2 branching last month.
If you're not already familiar with Mesa 9.2, there are many performance improvements and many new features to this important open-source project that's up to 1.3 million lines of code. However, it's not yet in a good state for all Linux users especially with its GL3/GL4 support.