They would probably be fine as a replacement starting battery if you had a dedicated accessory battery or were careful about not drawing it down. They don't tolerate discharge as well as a lead battery.....and to Cummins point about charging, they can be a little finicky with that......at least in my experience. I had one on one of my bikes, and it was fine, but my voltage regulator went and it discharged. I couldn't get the charger to charge it up...wouldn't recognize the battery as good (maybe it wasn't). I had to trick it by putting it on a normal battery charger to get the level up a bit and then switch to the lithium charger. It charged fine after that. That was my fault though.
Sounds like you already have them so give it a go....I just wouldn't recommend them for this application if you were considering buying them. Their biggest advantage (weight...which is the reason I bought it) isn't really much of an advantage on a UTV....not for the cost anyway. It's cool that you can mount them pretty much anywhere, but Honda has a spot picked out already that works. I'm not sure about the warranty thing.....Shorai (for example) lists them for a P1000 so I would think they would warranty them, but who knows. I'm sure there are others with more experience than I with these....maybe they'll chime in. @ohanacreek knows his way around a battery....and I think he did the warranty leg work mentioned above. good luck.