XM-L2 U3 + FET DRIVER OR MT-G2 Q0 + DQG TRIPLE BOOST DRIVER?
too excited to use minuscule letters
One vote for MT-G2. 
My vote for MT-G2 and FET driver, with 6x18350s powering it. 
For that I would need 4 more 18350s 
The SR52 has a really nice reflector for the XM-L2 though, especially for its size, so it would almost be a shame to use it for an MT-G2. If it were my light I'd keep it an XM-L2, but if you're set on an MT-G2, you might as well go all the way!
My initial planning was a dedomed xm-l2 but, as always, mt-g2 is an option.
I am thinkig keep the stock driver for it’s charging fuction and add a fet driver for the rest.
Possible?
Yes, you can even strip everything off the stock driver except for the charging circuit and even retain the charging light indicator.
You are always helpful Richard!
If I remove just the inductor, will I have parasitic drain from the stock driver? Will it be in idle/standby function?
I have two of Vinh’s SR52 with the XM-L2 dedome They do around 250k that’s only running at 4.2amps(one of my favorites run forever on high heat no issue)—Go with the XM-L2/Fet
Then get one of those $50 SR51 and put your MT-G2/Fet in—I have one I did does 10amps not a bad mod either OP reflector is like it was made for the MT-G2
I'm not sure, since I didn't even try it. I knew I wasn't going to use it again, so I didn't bother, I was just concerned with leaving the charging circuit functional. I'm sure that there will always be some standby drain from its circuits, although not much.
The stock driver looks like a very high quality buck/boost setup, using good components and set up without cutting corners.
Crazy idea: can i combine the fet driver with a 5v charging circuit?
http://www.ebay.com/itm/271539046253?redirect=mobile
Yes, but I'm not sure why you wouldn't just use the built in one. It is high quality and is already in place.
True, I have built a cheap 18650 charger with those TP4056 based boards and I would never put them inside a flashlight or anything expensive, they explode when polarity is reversed.
Wow :~