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The worst situation is when someone has snow blown/thrown their snow on top of the hardened snow, very unstable up front."
You may find that there are no tires or bikes that actually handle that stuff well. I hope I didn't post this already, but I was rather disappointed to find that the big-ass tires don't handle deep snow that much better than the skinny ones -
http://www.bikeforums.net/showthread...rathon-Winters
For the Marathon Winters, I've found that 35psi is usually the best for bad road conditions. Pressures above that (like 60psi) I only use for road riding where the streets are clear to the pavement, just don't want to hit a rare patch of ice and go down. Running them lower than that, twice I felt the tires actually road worse at less than 35psi (less grip and predictability), once on a very rutted and frozen trail I think about 15psi was a better ride...but again, twice before that I thought it was worse, so go figure.
You'll find that no matter what tire it is, bikes are always slow in the cold when riding on top of snow. My road bike with road tires actually goes about 1mph slower when it gets to be 40 degrees versus the 70 or so it usually is. Many, many other people have commented about the same thing. Whatever the reason is, it probably just gets worse when it gets down below freezing. And any tire always seems to be slower on snow - it's not a solid surface where all the energy goes into propelling you forward.
At least that's my theory - what I know is that switch from 28c tires in the fall to 35c Schwalbe Marathon winters @ 60psi made me lose about 1 mph when I tried riding them back to back. But once it gets cold and there's snow on the ground it's a much bigger loss - I just don't think it's a result of the tires.