IMO I wouldn't touch the 08, 09, 10 800. These motors had nothing but troubles from the get go and I don't have the confidence that it will be a reliable power plant. There were some that got a lot of miles with and without any fixes but there are way too many that went down and still do...
I can't explain the ratcheting noise however, I had the exact same type of problem with my D7. Great on the trail and seemed to slip in the powder. Turned out that even after I had my primary replaced and the sled gone over, my clutches were still out of alignment. Might want to take a look at...
Polaris 600 is a good strong engine. On a whole the 600 with the 155 would be a great package depending on the milage, condition and price. Handles well, strong power train, great sled.
I would do some digging on some other posts around here. I'm sure someone else has already done some clutching to make this sled pull harder.
A great resource if you are looking to understand a bit more and maybe make your own changes is Olav Aaen's Clutch Tuning Handbook. Pretty sure they...