When running the open-source Nexuiz game, which is also one of the more demanding Linux games where Mesa is good enough to handle, there weren't any major performance changes here for the five ATI/AMD graphics cards on the R300g and R600g drivers between Mesa 7.11 and Mesa 8.0-devel. The Catalyst driver remains many times faster, except for the Radeon HD 3850 graphics card that was acting up with Nexuiz for this game on the fglrx 8.94 release stream tested.
Nexuiz with a resolution of 800 x 600 where the game is more CPU bottlenecked, the performance with the Radeon HD 6950 "Cayman" graphics card was boosted significantly on Mesa 8.0. However, the other graphics cards did not experience any improvements with the newer open-source driver. Mesa 7.11/8.0 does run closer to the Catalyst driver at this resolution, but barely anyone runs at 800 x 600 with modern hardware in 2012.