Suction cup mounts

As I mentioned in a different thread, I use a PNA with XCSoar in the
cockpit. I have been mounting this with one of Paul Remde's big suction
cup mounts, and it has worked fine, only rarely coming loose in flight
(as long as I lick around the edge when applying it). But that's been
in thermal flights probably no higher than 7k ft. If I go higher
(oxygen altitudes) will it not work? Seems that is should be a problem
because it works off atmospheric pressure, and up there, there's only
half as much. Anybody have experience with this?

Ed