The results of Mesa 8.0.2 vs. 8.1-devel Git as of May aren't too interesting, hence why the results weren't quick to be published on Phoronix. Those interested can find them on OpenBenchmarking.org or facilitate your own comparison with the Phoronix Test Suite by simply running phoronix-test-suite benchmark 1206129-SU-INTELIVYB10. Benchmarking was done from the Intel Core i7 3770K with its Intel HD 4000 graphics (IVB / Gen7).
There isn't much to see in the way of Ivy Bridge performance improvements on 8.1-devel at this point, although there is better OpenGL coverage and other improvements. There's more performance improvements to be found at the moment for Ivy/Sandy Bridge hardware by upgrading to the Linux 3.4 or 3.5 kernels. The newer kernels do offer some Intel graphics performance improvements thanks to updates in their DRM driver. However, the Linux driver right now does still remain slower than the Intel Windows driver and is far behind in OpenGL coverage -- OpenGL 3.1 isn't even expected until 2013 in Mesa.
The 1206129-SU-INTELIVYB10 results.