Going back to June on Phoronix there have been Ubuntu Mir/XMir performance benchmarks. Up to this point these benchmarks have only been done with Intel and Nouveau graphics drivers, since the AMD Radeon driver has been rather problematic with XMir. Sadly, this still appears to be the case.
With the Radeon graphics (using R600 Gallium3D), it was very quick to hit issues with XMir compared to when the Intel or Nouveau graphics driver was in use on the same software base. The Radeon issues I encountered with the XMir Xubuntu image appears to come down to damage tracking when dealing with text-based programs like the terminal or gedit. When entering text, it usually takes an extra character being added/removed (or other change to the screen) for the character(s) to actually appear and some other text-related issues with XMir on Radeon. It was a big annoyance even with the limited manual text input needed for running the Phoronix Test Suite.
Anyhow, I was curious to see how the Radeon OpenGL and 2D performance compared with and without XMir on Xubuntu 13.10 Saucy. I ran some benchmarks similar to the Nouveau XMir Xfce tests. The new Radeon tests using the AMD A10-6800K APU can be found on OpenBenchmarking.org within 1308071-SO-RADEONXUB42.
Embedded below are just a few of the 2D/3D benchmark results showing the Radeon performance hit with XMir on the current (X)ubuntu 13.10 packages, click the aforelinked result page for all the system hardware/software information, system logs, and more test results.