fft - might not be a good idea to use the o-ring there - you want the LED more into the reflector, not further out.
MT-G2 Upgrade w/stock driver (CNQ group buy Y3 w/1 extension)
I added an 0805 size R100 resistor, swapped the LED out for a MT-G2 I had laying around (not sure of bin - think it's an original IOS version), nice neutral tint. I measured 4.6A at the tail with two protected KP 4000's, getting a nice 2186 OTF lumens @30 secs, even though the reflector is rattling (loose).
Then swapped the lens for a UCL/p I had, them measured 2,329 OTF lumens @30 secs (it loses bout 120 lumens from start). The UCL/p is a little thicker at 2.25mm, while the stock lens is 2.02 mm, so less rattle now but it's still there. I screwed down the MT-G2 Noctigon, used GC Extreme under the Noctigon and sanded surfaces smooth, carefully got it perfectly centered, and have the reflector sitting directly on the edge of the MT-G2, extended the wire solder pads out by using copper extensions, so the reflector sits pretty low. Also I kept the stock 22 AWG wires, but probably should use 20 AWG. 22 AWG wires in both springs.
A better fix would be to raise the Noctigon by reflowing it onto a copper shim - that should get the reflector sitting tight on the MT-G2, maybe use some kapton tape to protect the MT-G2 edge - I think this is another risk factor of having the reflector sitting directly on the edge of the MT-G2 emitter.
This needs more testing, it does get warm pretty quick as expected, and probably could benefit, and survive better by potting the driver, as 18sixfifty's thread "Are you on Pot?" describes very well. Didn't measure throw, but in testing it outdoors, it looked pretty good against my SupFire L6 with a XM-L2 T6 4C and a BLFDD driver, but throws a big spot of course.
I'm kind of impressed this was so easy - no idea the stock driver would so easily support an MT-G2, just hope it's reliable - didn't check for hot SMD's on the bench... I'm glad this capability was pointed out earlier in the thread.
Edit: measured throw: 43.5 kcd measured at 5 meters
Note: all my lumens and kcd measurements are my own taken on a older LX1330B meter - kcd taken indoors and lumens in a PVC constructed lightbox, accuracy can be freely debated (CYA all the way...)