In my research I only found people using Gnarly on XTs, but there were no listings on FMF for fitment so suspect that they are using RR pipes. Small market here (New Zealand) but was a listing on a site for FMF Fatty for XTrainer.
Super easy install. Bit bulbous but comes with the territory.
Stock jetting and wow. Seems to have same bottom end but just opened the midrange to top. Can lug down in 3rd, and then a second later pop the wheel over a dip. Much more fun. Need to ride it in more open area for comparison to my old GG300. Also have a couple of needles due, but it seems to suit stock jetting better than original pipe which was fluffy on steady throttle.