As a new reference point for those desiring some fresh 2D Radeon benchmarks, there is this OpenBenchmarking.org result file. I tested an HD 5830, HD 6870, HD 6950, R9 285, R9 290, and R7 370. EXA continues to be the default acceleration means for pre-GCN hardware while everything from the HD 7000 series and newer is dependent upon the generic GLAMOR 2D acceleration over OpenGL via the RadeonSI driver.
Tests were done on Ubuntu 15.10 with the Linux 4.5 kernel, X.Org Server 1.17.2, Mesa 11.2, xf86-video-amdgpu 1.0.99 Git, and xf86-video-ati 7.6.99. The R9 285 performance is rather poor in this tests as the AMDGPU support seems to have regressed with Linux 4.5-rc3.
GTKPerf and QGears2 were run for the 2D benchmarks today. Sadly, I haven't seen any updated test cases for GTK3 / Qt5. So if you are interested in Radeon 2D performance, go see the results on OpenBenchmarking.org.