Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
With there being experimental XMir-based Xubuntu 13.10 images available for the Xfce desktop spin and a request going out for testing, I ran some Phoronix performance tests to compare the XMir performance penalty for 2D and 3D workloads.
With Ubuntu's Unity desktop there's already been lots of XMir testing - the X11 transition layer to allow traditional X.Org applications (and desktops) to run atop a Mir Display Server in the same way there's XWayland to Wayland. The Xubuntu developers are currently deciding whether to ship XMir or just a straight-up X.Org Server environment (as is the case in current releases) for the 13.10 release in October.
Of the XMir Xubuntu 13.10 experimental images I have AMD/NVIDIA/Intel graphics tests forthcoming to show the current performance overhead of having this extra layer in the mix.