Don't know about the Nav mount, but the standard 660 mount does not look like it should be safe, but is. Did 400 odd miles of road round the Pyrenees last year (and lots of off road since) and never had a problem. Not safe from nicking, so tend to wipe it off into a pocket when leaving the bike un-sighted, but I guess I'm too tight to splash out on a TT locking one!
cheers
Markie