I'm 6'1", 210 lbs and ride a '09 M1000 162. On flat ground, the 1000 really doesn't have much over the M8. On a hill, there is no comparison. I ride with a lot of new sleds, cats, D8's, XP's, King Cats and older Edge's. The M10is a silent killer. It can be gentle down low but when you hit the gas it pulls like a tractor. I've got the SLP pipe and sno pro intake which removes 24 lbs on paper (16 lbs with exhaust and 8 with intake). The front end isn't heavy! We rode 4 days last week in the Jefferson area and when climbing on the M10 you look for the best place to crest the top of the hill. On the 800's you'd look for the best line to come back down. The M10 handles just fine in the tight stuff and works great in the deep snow. Your big enough to handle the extra lb's that is has over the 800. The difference in top end power is enough to upgrade to the M10.
I'm not bashing on the 800s either, they are great. The M10 just has more beans in the pot on top end.