Which one?
Of the 4AA Pro Poly models, the only one where I noticed a difference in output was with the incandescent lamp assembly. The output from a fresh set of alkalines was notably better than what I got from freshly charged NiMH or NiCD batteries. Never noticed any difference with NiMH batteries in either the 7LED or Luxeon models of that light, if anything, it's pretty much a consensus that you'll get a little longer run time using the NiMH batteries over the alkalines. Not sure that using NiMH batteries would necessarily nullify your warranty, I don't see anything in Streamlight's warranty that would specifically void your warranty if you used NiMH batteries, however I don't see where they would be liable if your NiMH batteries leaked any more than they would if alkaline batteries leaked. The only Streamlight product that had an explicit warning about using specific brands of batteries are those Streamlight products using the Lithium 123 batteries; there's a bright red sticker on the package with that warning.