And what kind of mower do you use? Am I being optimistic thinking about mowing 17 acres of semi-hilly pasture with my Sportsman 800?

I regularly keep a 20 acre area around the houses and barns mowed with our P360 and a Swisher rough cut mower. We bought the Swisher with the larger engine and electric start which I feel was a good option and worth the extra bucks. Not a real fast operation but this combination takes care of any brush and tall grass that gets ahead of me. I have also used this combo to knock down fair sized sage brush and heavy grass along the road right-of-way. Don't think you would have any problem with your pasture especially if you set the mower at about a 4" cut height.

The Swisher mower I have has been great, it's a 10.5 hp motor with a 44" cut and it'll go through some deep grass before it ever dies. It's a pull behind that I pull with my tractor, I can pull it to the side so I cut like 75-80 inches at one time, 36 with my tractor and 44 with the Swisher. You should find a cheap tractor and do this it's amazing how much time it saves me. If you do use one with your 800 you should use low gear, also find the widest cut avalible to shorten the time it takes b/c the hours will start adding up on your 800.

My dad mows around 15 acres with a vinson and a swisher 44" rough cut and has for years.
My friend had lots of axle problems with his 700 polaris while mowing his place, he never had problems mowing with his vinson and he doesn't have problems mowing with his bombadier now so I am just assuming it was a polaris thing. (he still has the same swisher mower also, in case your looking for a mower to, check out the swishers)