When running Nexuiz at 1024 x 768, even for these relatively powerful GPUs, the frame-rates were too low to result in a playable experience with the R600g driver. Even Intel SNB graphics and their classic Mesa driver tend to deliver better performance for Nexuiz with their latest round of optimizations. The Catalyst driver is ten times (or more) faster for most ASICs with this game.
Obviously, at higher resolutions, the Nexuiz performance is more toxic for the open-source drivers.
OpenArena is playable for the Radeon HD 4670/4830/5750 graphics cards at 1024 x 768 for this game, which is not too demanding relative to modern titles... The Radeon HD 5450 here on the open-source driver failed to breach even 30 FPS at 1024 x 768 while the Catalyst driver was hitting nearly 120 FPS. The other graphics cards were approaching 400 FPS on the proprietary driver.