I am picking up my 2015 XMR 800 on the 16th. I see ALOT of redundant exhaust questions, and I also see vforcejohn is a definite asset to everyone, and Muzzy is a definite contender. MY question, instead of which exhaust is best, would be to ask how many XMR's out there, are running LTE slipon singles, with no programmer, and what are you seeing?
If you don't plan on buying a tuner, I would suggest letting the stock exhaust on. Its tuned to run the stock exhaust and it can't adjust to the new pipe (whatever brand you pick). The tuner simply adds the needed fuel to get the air fuel ratios corrected.