Would it make sense to use our proximity to the surface of the geofence as an input to limit our max groundspeed?  That would potentially prevent us from arriving *at* our geofence at a high velocity.

Basically, you can go as fast as you want well inside the geofence but as you get closer, we proportionally attenuate the max allowed ground speed.  This would at least mean that if we ever punched through the geofence that we would do it at a low velocity which would (hopefully) limit our excursion.