Reading all these threads on the 850 kinda make me chuckle. The 850 was introduced to give the mountian boys a little grunt at elevation. The p-drive increased the acceleration and made the sled fell faster. You guys have to understand it's only 10 hp. more than the 800 etec. 10 hp. does not make the sled 10 mph faster. As far as speed increase usually rule of thumb add 5 hp. gain 1-2 mph. Don't get caught up in all the magazine propaganda. The G4 was designed to change the riding experience as with the engine position and seating. The 850 was a bonus as it was designed to fit the chassis.
The 850 is not a lemon, it is what it is. Variability is always a factor but in the end I don't subscribe to the theory of miracle sleds and complete dogs. Machine tolerances are far to precise these days for widespread differences. In more cases it is environmental conditions, difference in sled setup, track selection, rider ability, the list is endless. In this case as others have already stated track makes a massive difference and the shift to off trail leaning tracks, i.e. 1.5" lugs and larger just provides more disappointment for on trail use when the effect is scrubbing off top end speed. Want larger lugs? Accept that you will trade top end performance for off trail capability.
Also, my experience with the 800 is with a 2013 and a 2016. The 16' was a big improvement in overall performance from the 13'. Don't ask me what BRP did in the 2015 and later 800's but they all ran so much better than any 800's prior to that. As has also already been stated the 850 was not designed to deliver a massive top end gain with that additional 10HP. It was meant to deliver better performance across a wider mid-range band which is where the 800 was not a great performer. The 850 completely solved the mid-range lag or laziness for lack of a better term. The G4 chassis brings a lot to the table in the handling department. It is a big improvement overall and combined with the new 850 motor has been an overwhelming success for Doo with most everyone that has tried them. Not many dislike them.
^^^ Nailed it.