Milkyspit has done M6 mods. Others have probably done the same. Mods being more then drop-ins.
The only drop-in that I know of is the tower drop-ins and those don't seem to have much heatsinking.
A Seoul P7 is essentually 4 Seoul P4 LEDs in a single packet, the MC-E is similar as it also fits 4 Cree XR-E dies in a single packet. The MC-E allows the LEDs to be wired differently though while the P7 has all the dies wired in parallel. The MC-E is also smaller then the P7. They provide more light then their single die cousins but have a larging emitting area so they need a larger refelctor to have the same beam pattern as their smaller cousins.
The refelctor hole can probably be enlarged but that won't look to good with the incan module back in there. The space in the M6 also doesn't seem to allow too much heatsinking with a tower module.
I remember seeing 7 Crees fitted into a M6, presuming Q5, 1 amp to each LED, 80% driver efficiency, and 70% reflector + optic efficiency(probably higher in reality), that is 1100 something lumen from the light(1600 lumen at the LEDs) and a runtime of over 40 minutes. Less output would amount to more runtime of course(500 lumen out the front would give over 2 hours of runtime). Beam pattern would be different though.