You're right. "you'll have to play it by ear"
I have 4 regular n.o. fluorescents about 2 inches over water, but they are not MH
I would try to shield them somehow ? I don't know. why not put a sliding glass cover on the tank.
as for your powercompacts not touching the bottom of the tank I assume this is a 24" high tank right?
Or just save a hell of a lot of work and go here
http://www.hamiltontechnology.com/sh...87f2cd241fe152