No limitations on speed, but "snow mode" is a bit hyperactive about applying brakes to control wheel spin and this can impede forward progress under some conditions. Turning "VSA" off with the button on the dash fixes this, but isn't recommended at highway speeds. If you are drift bashing at highway speeds, I guess you're on your own! Really, I tend to just leave it in "normal" mode and it does fine with winter tires. Might be different on all-seasons.
I think they still get lake effect snow in Buffalo if you feel a desperate need to play in the snow. We have had some teasers, but then it went away.