First question is why do you need a can for the 850: weight saving, making more noise, increased performance?
All after market cans offer some weight savings, most aftermarket cans make more noise, most aftermarket cans don't increase HP, the pipe creates HP. Check out Rich Daly Dynoport and Olaev Aenn hand book.
The stock can offers the best compromise for the above at no cost to you and no additional costs to replace melted plastics etc.
I do not ride groomed trails, I ride in the Valemount, McBride area when in the mountains, between Nordegg and Edson in the foothills and cut lines and lakes in Northern Alberta. None of these areas are near towns or villages but I have decided that making more noise than a stock machine is not for me. Listening to the narrative that "mountain cans are okey in the mountains makes me chuckle, really?
What makes people think that the mountains are for sledders only? Heli skiers, snow boarders and down hill skiers as well as hikers all share the same areas not to mention the wildlife. Just ask the folks in the BC snowmobile clubs that have to constantly fight to keep the remaining snowmobile areas from being closed by the government. My rant for the day, its your money buy whatever you want to. JMO