860 vs 800
I know a couple great guys that can tune sleds (one being Lynn Felker) that have messed around alot trying to get an 860 better than a 800. Only advantage was the first 30ft. the 860 would pull ahead, other than that there the same.
860's are a little better for turbo's cause of more torque off the bottom. From experience in tuning they are harder to get dialed in at high elevation than the 800, but once you get them, they are better for boondocking and in and out of the throttle situations because of the slightly more broad torque band off the bottom. Other than that, 860 vs 800 overall hp is the same, if you think your goin to get more top-end hp on an 860, your pissing into the wind.
When going from 2K ft. to 9k with my sled, it was amazing how much difference in jetting to finally get it to work. 450 mains, 6DH4 jet needles, power jets 2 1/2 turns. at low elevation. 9k is 350 mains, 6DH3 (3rd slot), power jets about 1 1/2 area, depending on temperature. Use slightly less fuel than a stock 800 for general boondocking, deep snow and hillclimbing you start drinking fuel. 11.6- 12 A/FR WOT on 1 min+ hillclimbs.
Really Hope you get that sled dialed in! It will be a runnin sled forsure!