Nice build Itinifni 
Mags still have a feeling of nostalgia IMO. Take a true American classic, mod it, and you have yourself a sleeper light.
With 4 Nimh cells used to power a AMC7135 based driver you might run into some trouble. Excess Vin above the LED Vf will be dissipated as heat through the larger ground tab. According to Match’s testing a XM-L2 at 6A wil have a Vf of roughly 3.9v. With a Vin of roughly 4.8v the driver will be dissapating (4.8-3.9)v X 6A = ~5.4 watts of heat. Take a look at TechJunkie’s build over at CPF for further details.
He had to resort to using resistors to burn off excess Vin (Vbatt) in order to prevent the AMC7135 chips from overheating and throttling output. Your potted driver may or may not be able to deal with the heat, only one way to find out.