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. Xfce presently doesn't support Mir directly nor has upstream said whether they will, though Wayland support has been talked about in the past and they are GTK-based where there already is the Wayland support when using GTK3.
For those wanting to see the Xubuntu XMir performance results using the newly-released ISOs, all of my test data in full is on OpenBenchmarking.org in 1308067-SO-XUBUNTUXM33. Embedded below are just a few of the graphs for this Nouveau driver testing from a ThinkPad laptop.
Yep, there's still a noticeable performance penalty in 2D tests when dealing with XMir.
Some X11perf tests see little overhead for the micro-benchmarks.
The performance impact of XMir when having to deal with OpenGL Linux games is still very noticeable at this time with the latest Ubuntu 13.10 "Saucy" packages.