On grass I've used turf mode and run some in high. I think on snow that's not so deep on the lake, high would be fine as well. For me, I'm not usually going that far anyway, and I'm probably a little over-protective of them ($$$). I've seen documentation that says that 25mph (40kph) is a safe maximum speed, particularly if you're on the flat with enough lubrication (snow).
Power-wise, I was actually surprised and impressed with how little the tracks seem to drag things down. The deepest snow I've been on so far was only about 14", but the machine moved thru it seemingly effortless. I was nervous that the 570 might not be quite enough power, but it works awesome.
Slush-wise, we didn't have much deep stuff last year, so I've only heard the stories and seen the videos. I think the tracks will be WAY better than a snowmobile, at least getting moving, but its so much bigger and heavier of a machine that just having a couple guys muscle it to get moving is probably not going to happen if you get caught sitting in 12" of slush. Again, my (untested) plan is that my winch will be my friend if that ever happens.