While Xonotic at 1280 x 1024 had issues in composite bypass, fortunately it didn't at 1920 x 1080. The latest Mir packages still cause some performance loss over a pure X.Org Server, but it's not nearly as bad as the state of Mir/XMir yesterday in Ubuntu 13.10.
The new Mir packages were also beneficial in eliminating some of the performance loss for the GL3 GpuTest workloads too.
The Mir/XMir performance is moving in the right direction and eliminating more of the performance overhead of having XMir in the rendering stack, but it will be interesting to see how the real Mir performance is when Mir-native games/applications begin to appear in 2014. There are also other features to be worked on for Mir aside from closing the performance gap.