I have the 20hp Kohler Magnum in mine. With a hard tire like the Terra 26" I haul tail on the trial with my butt well off the seat, knees bent. You will smoke any 16hp on any decent dry terrain. 30mph on flat dry stuff and grass.
At a crawl (how I drive in the rough and off trail), I don't think there is much difference between the different hp of motors. I have seen a little wee briggs on another rider's old orange 8x8 argo with little bitty muck-donuts do anything on the trail, including mud wading.
If I bought a new engine, I would get an 18hp or 20hp Honda, if I was made out of money I would get one with efi. Kohler parts are really spendy, that's their main drawback in my opinion.