The saving grace with the Arc LS design is that there's some flexibility in how deep the reflector can be... the entire circuit/emitter board is "floating" on a threaded ring inside the light and can be move forward or backward a bit with no trouble at all. Because of this, I would guess the Q3 reflector ought to fit nicely by the time it's opened up for use with the Cree emitter.
The reflector would still need some material removed from the outside, though, to get it narrow enough to fit into the Arc LS head... but there's lots of meat on them bones, so no worries.