We have talked about Mir's support (or there the lack of) composition bypass in the past, but with today being the Ubuntu 13.10 feature freeze, the support was finally committed to its mainline code-base. The merge to trunk happened with this commit. When fullscreen games/applications are run, Mir can ignore compositioning and render directly to the display hardware.
Intel's hardware/driver is best supported with Mir right now (and what most of their developers are using) while the Radeon support is in good shape and the Nouveau driver is in good shape to a lesser extent. No composition bypass support has yet to land for Mir on Android.
For more details on the performance work going on for Mir, read this lengthy blog post by Thomas Voß. In that post the Canonical engineer covers the performance work going on for Mir, their performance testing, etc. Most of their benchmarking right now is being powered by the Phoronix Test Suite.