I modded a Sunwayman C20C with an MTG2.
The Sunwayman C20C is a relatively small single 18650 light. It's smaller than a Zebralight SC600. The driver I used maintains modes and can ramp up and down. Works pretty well actually. I'm running it at 3.04 amps on two IMR 18350 cells.
The result:
(1) VERY bright. Probably around 1700 lumens out the front. Should be around 2100 at the emitter. Amazing output for such a tiny light. Much brighter than comparably sized XM-L2s. Brighter than an XM-L2 at 6+ amps on copper, but with less throw.
(2) VERY floody beam. The emitter is HUGE compared to the size of the small orange peel reflector I'm using. The beam is enormously floody, but does have a very wide hotspot. Point this at a wall in a house from close range and the entire wall lights up. However, for distance viewing it doesn't have much throw. My much smaller modded 3.04 amp XTAR Wk42 with a single XM-L2 has significantly more throw.
(3) BEAUTIFUL tint. MTG2s always seem to have awesome tints. Very pleasant neutral white.
(4) Heats up very fast. The head of this light gets hot fast. Not as fast as my other modded C20C (triple XPG2 with FET driver, pulling around 7.4 amps), but still fast enough that it becomes too hot to touch comfortably after about 1 minute at max. Runtime at max power with my IMR cells should be about 15-16 minutes, but the light would burn up from overheating long before then.
I think you aren't seeing MTG2s in small form factor in stock lights because:
(1) they require 2 cells in series or some kind of boost driver,
(2) the cells required are IMR 18350 or maybe IMR 16340, which aren't as nearly common as 18650, limiting the market.
(3) don't really take off till around 3 amps which limits performance on small lights due to limited battery size,
(4) produce a very floody beam, due to the very large low-luminance LED die.
XM-L2 seems more optimum for such a small light.