We frequently do RadeonSI Gallium3D vs. Catalyst benchmarks as we see how the open-source AMD Linux driver for the HD 7000 series hardware and newer does against the Catalyst binary blob and also the R600 Gallium3D driver that supports the HD 2000 through HD 6000 series graphics cards and that driver is already in a mature state. In our most recent tests we have found the RadeonSI Gallium3D driver on HD 7000/8000 series GPUs to do well against Catalyst, but generally the Catalyst driver still reigns superior -- especially if considering its OpenGL 4.x support and much better working OpenCL compute support.
Phoronix reader and prolific forum contributor darkbasic has concluded in his most recent personal testing that Radeonsi is awesome, beats Catalyst! With his AMD Radeon HD 7950 graphics card he was using the Linux 3.15 kernel with the yet-to-be-merged PTE performance DRM patches and enabling HyberZ (R600_DEBUG=hyperz) while using LLVM 3.5 Git with Mesa 10.3-devel Git, the performance is really great. He's also using Keith Packard's glamor-server X.Org Server 1.16 RC2 code while comparing this bleeding edge open-source code against AMD Catalyst 14.4.
All of this very latest open-source RadeonSI driver code yields it being 14% faster than Catalyst with Xonotic, 4% faster than Catalyst for OpenArena, 76% the speed of Catalyst for Unigine Heaven, and 62% the speed of Catalyst for Unigine Valley. You can see more of his performance figures via his personal blog. This Italian Linux user was using our Phoronix Test Suite and OpenBenchmarking.org software for test automation and reproducibility of the results.