While it shouldn't surprise many Phoronix readers, upgrading your Mesa and kernel on even a few-month-old distribution like Ubuntu 15.10 can mean big speed gains. On Ubuntu 15.10 it's easy upgrading your Mesa and other user-space components (libdrm, the DDX drivers, etc) along with the LLVM back-end by using the Padoka PPA. If you want to also upgrade the kernel (primarily for AMDGPU users and other newer features found since Linux 4.2), there is the Ubuntu Mainline Kernel PPA.
From a Radeon HD 6870 and Radeon R9 290 I ran some comparison tests of the Ubuntu 15.10 stock performance (Linux 4.2 and Mesa 11.0.2) compared to Mesa 11.2-devel Git and Linux 4.5 Git using the respective archives. All the benchmarks, of course, were driven via the Phoronix Test Suite.
Even for the older Radeon HD 6870, the open-source Radeon driver stack is still maturing and getting faster. You can see all of these comparison benchmarks via this OpenBenchmarking.org result file.
With the R9 290 on RadeonSI, those gains are even more profound. You can see those benchmark results via this OpenBenchmarking.org result file.
Stop by our forums if you run into issues with driver upgrades or are wondering how to upgrade your open-source drivers on another Linux distribution.