The HID mod isn't more expensive, and if runtime IS relevant, it's the perfect solution.
With the MB reflector, it's only about 70% effiecient, even less than a Thor, because it has to be very resitant to UV light, which the short arc throws out a lot of. It has some rare earth or other rare element coating that makes it expensive (but perhaps not as stupid expensive as the Maxbeam is as a package-heck, even the regulated charger is like 400 bucks, which would make even Surefire blush). Another thing is, it's just not that bright. Thor HID is much brighter and if you were using the light to look for someone, the Thor is much much much more useful. The MB just has a tiny little spot which you need binoculars to look at (if and only if, the night is clear). Finally, the Thor HID is better than the X990 (having both, the small reflector of the X990 is not as good).
Eh, so what am I trying to say, er, well, the Thor HID is probably the closest thing to a homemade Maxabeam (esp, if price is a consideration) this side of the Blitz 240 (but that's another story).