The 2011 JD 825i, just announced and becoming available, is supposed to be capable of 43-44 MPH.
If you're doing recreational trail riding, you'll likely be operating under 25 MPH most of the time, but the ability to accelerate is VERY, VERY important because you're constantly slowing down and speeding up.
This also plays into the ability to maintain speed on very long slopes and hills. I'd venture to say that a sport-utility will likely climb the same long slope at about twice the speed that a Mule, RTV or the earlier JD Gators will.
The difference between 10 MPH and 20 MPH on that slope then becomes quite significant. For example, my Rhino will only go about 25 MPH in Low Range, and after some tweaking about 41-42 MPH in High Range -- but I can accelerate when going up 25 degree slopes in Low Range, even when loaded. In High Range, it takes a while to accelerate from 25 to 40, but typically I'm only running that speed on roads any way. It's the 0 - 25 power band that's so very important...