I have remounted my Magellan, Valentine, and alarm display box off the windshield. California and Minnesota (probably others to come) have laws on the books prohibiting anything from being mounted on the windshield (with suction cups).
The Magellan and Valentine are on a permanent mount on the dash. Signal is good, but I am going to use an external antenna, not because I need it, but just because....
I will post a picture sometime this weekend.
One disadvantage of mounting the radar detector down low is that it reduces it sensitivity, esp to laser. Not that even top end detectors like the Escort 8500 or V1 can detect laser that well anyways, but any advantage helps.
If you believe the glue holding the rear view mirror onto the windshield is strong enough, I'm sure you can jury rig some sort of ziptie/attachment between the mirror post and the mounting bracket for the radar detector. That would technically make it not suction cupped to the windshield and still be high enough to foil those CHP officers hiding behind the freeway overpass.
I have been doing a research on radar and laser detectors for a while. Most of the people mount radar detectors high on thier windshield, however this is best location for most types of radar but not Laser. Laser is mostly aimed at your headlights or license plates because they are highly reflective. Mounting radar detector high will decrease your chance of detecting laser. Best place for mounting is on center of the dash and close enough to reach for it in case.