I hardly think that Mutter is the be-all and end-all of Wayland compositor performance. I could see Ubunu plumbing a re-moulded Mir back into their distribution further down the line (while keeping the rest of the GNOME environment). I'm not certain on the technical details, but I believe this should be possibly; for Wayland was certainly designed with compositor agnosticism in mind.

Edit: reading further, it appears that GNOME Shell is quite tightly woven into Mutter, which may well complicate matters.