Probably it will work just fine. Something like XFCE i am not sure. But XFCE (Xubuntu) could still use different display server until it upgrades to GTK+ 3 that is said will have Mir support. If Ubuntu will not switch to lets say KDE apps then i doubt GNOME 3 apps will have problems running on Mir. But this is just my speculation based on current info.I'm just wondering how/if GS will work under Mir.