I have had the uforce 500 since September of last year. Like you, my wife and I aren't much into the speed thing anymore, although it is nice to open it up when you get on a smooth section of wide trail. For me it has plenty of low end power. When in low and turn on the 4x4, it'll crawl over pretty much anything you want to go over. I don't do any extreme riding with it but have been down some pretty rugged trails, through wash outs, over logs etc and it more than handles it.
I've never had it overheat, not even close and we put on 140 -150 km (85 - 90 miles) on some days. We don't have the high elevations but have run on some fairly hot days.
It is also alot better on fuel than I though it would be. About 3 gallons for a full day of running.