Wow, that title sounds like keyword spam, eh? :D
I got a bag full of... whatever these lights are today. They were listed as 'X-Power' brand, I guess because 'Xiaozhi' is a bit, well, awkward. But what was in the bag?
After taking lots of measurements and poking at the calculator, it looks like the head can be shortened by .446", and that's with a real double-AR lens in front of the Carclo TIR (no o-ring, lens glued in with clear RTV - learned from previous experience the o-ring just gets pinched and squeezed out through the hole when assembled tight enough).
Stock overall length (OAL) is 4.612", with the shortened head OAL is 4.166". If the tailcap is converted to a twisty instead of clicky it can be shortened by another .405", for a OAL of 3.761". There's an extra .100" in the battery tube length if you use only unprotected cells, but that might be pushing things too far. It can be done though.
The unthreaded section in the front end of the head is .618", from the underside of the bezel/front face of the lens, back to where the threads begin. That can be filled in by adding a ~.130" thick aluminum disc to the top of the pill, and removing around half the pill threads, taking that area down to an OD that fits snug into the unthreaded bore above the threads (.810" ID). Still enough to move the heat, and yet keep the pill easily removable more or less like stock.
I'm still waiting on more parts to arrive, but I have enough here to complete 1 or 2, which is enough to start chopping them up into little pieces.