I have a AMD 5970. Default gallium3D are 40-80% of the catalyst drivers in the testing I've done, similar to the numbers phoronix gets.[1] If you're willing to put up with some glitches, you can enable the SB backend and bring it to 80-95%, especially at higher resolutions.
I have a steam account and several Humble Bundles. From my experience, the catalyst drivers always perform better and have fewer glitches. Many indie titles are unplayable with gallium drivers. From openbenchmarking, Unigine won't even run under the gallium drivers.[2]
What I'd really love to see is AMD commit to merging their Catalyst and gallium drivers/teams. Sure the current 1 or 2 AMD open source guys can get good results running Quake 3 derivatives. But without a full team backing them up, I don't see how they can test hundreds of games for glitches and performance regressions.
[1] http://www.phoronix.com/scan.php?page=article&item=amd_hd500...
[2] http://openbenchmarking.org/result/1109040-LI-AMDRADEON19