For some weekend benchmarks I ran some 2D performance tests on the AMD A10-7850K Kaveri APU when comparing the open-source driver to Catalyst. The open-source driver stack was in the form of the Linux 3.13 Git kernel, xf86-video-ati 7.2.99 Git, and Mesa 10.1-devel Git from the Oibaf PPA on Ubuntu 13.10. The Catalyst Linux driver tested was the Kaveri APU launch driver and marked fglrx 13.30.1.
More system details and all the results in full can be found on OpenBenchmarking.org via 1401178-PL-2DPERFORM38.
Overall, as you can see from all these RadeonSI vs. Catalyst results on the Kaveri APU, the GLAMOR-based open-source 2D performance continues to be quite slow compared to the proprietary driver. Fortunately, Radeon GLAMOR improvements are being worked on but they're not quite baked yet for Radeon HD 7000 series (and newer) GPU owners on Linux.