Do you have any pics of the driver with the pill out? I have actually shoehorned a Qlite into a bit smaller than a 16mm driver space and the inside edge was fine, the outside edge dinged up the side by side 7135 corners when I tightened the battery tube too far. The spring was too long, so I had to trim that as well… It was an AA format light. Works good and does not get as hot as I thought, so you should be OK with that one.
It will take some paitience to file the driver down, go slow and steady. I checked mine often and the ground ring may get thin, but it worked fine, the boards have a few ground VIA’s that keep it all together for the grounding.
I do not know that model of Streamlight, but if worse comes to worse, the driver may stick out 1/16” or so (max), you should be OK with a good driver spring for taking that up on the new battery…
I actually have that fasttech driver, and I got a bad one, so I did not get to see how good it could be. Fasttech refunded my money and I bought something else. It would be a good option as well, but on direct drive, check the heat when you run it.
To me, the difference between a S2 XP-G2 and an XPL V5 or above is minimal between them in a current regulated driver. No doubt the XPL is brighter, but not so much as the XP-G2 will be over the original emitter…
You could spoil him and put a Nichia 219 in the thing… Still brighter and colors would look great…
You can buy two XP-G2’s on copper for the cost of the one XPL, then you have another should something go wrong, or for another project.
EDIT:
Pics of the filed down Qlite in a Thorfire TG06


The circle shows the area that is worn from over tightening the battery tube, and I actually sanded the corners of the top two 7135’s, but no negative effect.
