I used the Powder River Precision kits on my XD-M's and like it very much.
Reduced take-up, pull, over-travel and reset distance significantly, reduced trigger pull weight, smoothed out the pull and makes for a cleaner break.
BUT, I never tried their Hellcat kit, so I can't say it will produce the same results....
Oh, warranty, after market modifications should only void the warranty on anything affected by the aftermarket change.... ...but I "think" the law only forces that policy on Auto-Makers and not everyone....
So Gun Makers may get away with saying any modification or disassembly/re-assembly past field stripping will void the entire warranty...
As well, if the problem is totally unrelated to the aftermarket modification, there is always the option to re-install the OEM parts and configuration to avoid any controversy when making a warranty claim...