I don't have a heated screen, so i will leave that answer to someone who does, but the reason you have to cut the dash padding away is most likely to reduce vibration as the the padding will make this worse than it is when screwed direct to the scuttle.
I prefer the scuttle mirror and it is essential if you have aeroscreens, but it does create a blind spot as its around eye level, you might find yourself looking round it from time to time
Regards john k
