Fine to have a XM-L in a flashlight, but it the quark 123 reflector is actually on the small side. The XP-G may be less efficient, but the smaller die means a better thrower, which is usefull. But ofcourse, it all depends on the use of the light.
XM-L isn't a direct replacement for the XP-G, it is very different. So don't expect the XP-G to be made redundant very soon 🙂.