As of yesterday I did a clean install of Ubuntu 13.10 at the last minute on the Gazelle Professional laptop prior to sending it back home to System76. With the Ubuntu 13.10 20130909 state I ran benchmarks in its stock configuration and then again when enabling XMir with the unity-system-compositor configuration. The benchmarks in this article aren't around 3D/OpenGL performance but just the 2D performance.
The Gazelle Pro has the Intel Core i7 4900MQ "Haswell" CPU with HD Graphics 4600. Ubuntu 13.10 in its current state is using the Linux 3.11 kernel, Unity 7.1.0, X.Org Server 1.14.3 RC1, xf86-video-intel 2.21.14, and Mesa 9.2.0. No other changes were made during testing aside from setting up Mir/XMir on Unity 7.
While earlier benchmarks have shown dramatic performance hits to 2D with XMir, the tests conducted yesterday showing the 2D benchmarks performing better in relation to the performance of a pure X.Org Server. To see these results, visit 1309109-SO-UBUNTU13143 from OpenBenchmarking.org.