Going back a while now Vadim Girlin has been working on shader optimizations for the open-source AMD driver. This optimized back-end hasn't been a huge focus among other AMD Linux driver developers since they are already set in their ways of using the R600 LLVM back-end, which is needed for their OpenCL/GPGPU focus.
Vadim, meanwhile, has continued improving his R600g optimization back-end and now it's been merged into mainline Mesa as an experimental option.
Per this commit, the R600 Gallium3D optimizing back-end for AMD Radeon HD 2000 through HD 6000 series hardware can be enabled via the R600_DEBUG=sb environment variable.
This new back-end was committed just hours ago and came in over the copurse of several Git commits. Benchmarks on Phoronix of the R600 "SB" improvements are forthcoming but until then checkout the 15-Way Open vs. Closed Source NVIDIA/AMD Linux GPU Comparison.