I think they have improved - the winter set I have on order (for delivery this week - yay!) are V speed rated, which is up to 149mph. So I think the temp issues must be resolved, you'd have to be insane to drive at >140mph in icy and snowy conditions!
Actually yesterday i was doing 180kph on ice/hard packed snow. Some of the roads here are in better condition in the winter than in the summer :)