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You certainly can make a 2015 perform but, it will cost you. I have a 2015 and it works great now after a 2106 36" front end (may be too narrow?) 3" track, 8 tooth drivers with a dropped chain case and a belt drive, MDS clutch weights, SLP stage II, etc.
I went on a demo ride of the new 2018 M8000 and after jumping back and forth several time, I determined I liked my 2015 better. It was lighter, more nimble and the power plant on the 2018 felt very much like a four stoke.
YUP, Me too, Thinking of setting my '17 Mountain cat to 37.5 or 38.5 stance for this upcoming season?