I am using a Garmin 478 with the GXM antenna. For I now I just have the antenna stuck to the top of the GPS, using a piece of steel with double stick tape for the magnet. Crude, but will work until I get a chance to go in the fairing. Only problem I have now is on bad bumps the antenna pops off
Those using the GXM and have the GPS hardwired for power be aware, the GXM consumes a "lot" of power. The antenna gets warm to the touch when on, that should be a clue.
So be aware and don't kill the battery on your 800+ pound bike
BTW, I put in an accessory switch in the "blank" spot in the fairing and use that for power, this way I can just hit the switch if I think I'll be long enough to be concerned about the current draw.