Josh Arenson committed today the start of a "performance tests" category for Mir, as part of their built-in testing harness. The only test initially added is the OpenGL ES 2.0 port of the glmark2 test. Running this test in Mir simply ensures the performance meets a baseline threshold for ensuring no really bad regressions make it into the Mir code-base for slowing down its graphics performance.
Of course, outside of the Mir code-base is a lot of testing going on for performance reasons, including our occasional independent XMir-based tests. Fortunately, with there now being a SDL2 Mir back-end we're closer to being able to benchmark natively various high-profile OpenGL games under X.Org, Wayland, and Mir without any interference from the XWayland/XMir compatibility layers. Albeit, we still don't know when Mir + Unity 8 will officially ship for Ubuntu desktop users.