Mine's a Tom-Tom "Go Live 820" - the much-hyped (and then banned) Top Gear edition. It works fine in either under-dash position or stuck to the dash itself - doesn't drop signal.
However, as previously mentioned, I can hardly hear the voice-over when it's mounted underneath. Despite this, I soon got sick of Jeremy Clarkson's belligerent bellowing, and switched to the polite young lady whose dulcit tones are hard to hear even when up on the dash!
Frankly, I'm amazed that the radio-plate mount you describe works for you. Are you very short? I can't see the radio in my car at all - unless I virtually lie down
