I have a 2008 620 xuv gator it seemed a little doggy at first. It has 100 hrs on it now and its deffiently better. I did do a few simply changes to it. I purchased a K&N filter stock #RD-0470 it helped. Thats probably one fault with the machine is were the filter is located it sucks a lot a dirt in and the filter must remain clean or you will lose hp. I rerouted the intake of the filter housing to under the passenger seat and covered the opening over the battery under the seat and I don't have to clean the filter much. It gets plenty of air to it from other openings behind the seat. Another thing to keep in mind is don't use 87 or 89 octane use 92 or higher. Higher octane has less moister and won't draw as much moister when the machine seats for awhile. I notice a big difference with the higher octane. I also replace the ngk spark plugs with E3 sparkplugs it seems to fire better. These changes didn't make it run faster at top end but it made a difference at low end up hills and snapper.
I put ITP terrea cross tires on it they are bigger than factory, this did not change performace at all. I can tell you it has serious low end and eats some serious river mud. I haven't had it stuck. Also when climbing steep hills use low or 4 wheel drive. Alot a power is lost when tires start spinning.
Remember one thing all these new machines are great I have friends that have Rhinos, Rangers, Razors n etc. but each machine is design for different needs and pleasures.
dirty2