After the Intel kernel DRM tests, I ran some quick tests comparing Mesa 8.0 and the Mesa 8.1-devel Git state to see if there are any noteworthy performance boosts visible there yet in the usual stack of Phoronix Test Suite / OpenBenchmarking.org test profiles.
Aside from the performance though, Mesa 8.1-devel should introduce Intel Haswell support, GLSL 1.40 compliance and possibly OpenGL 3.1 support, and possibly other features.
There's still several months left until Mesa 8.1 will be officially released, but at the moment for Sandy Bridge there doesn't appear to be any notable OpenGL performance boosts.
The test results in full with several other OpenGL Linux games can be viewed on OpenBenchmarking.org. Separately, I also have carried out Mesa 8.1-devel benchmarks from Radeon Gallium3D for those interested.