I know people running 8lbs of boost on a Procharger, with no adverse effects on the motor.
As for the question about nitrous, I've been running a NX Ford EFI 100+ wet shot on my Mach 1 for over 3 years and 55k miles, with nary a hiccup from the motor! I have blown the rearend twice and been through 4 clutches and 3 sets of brakes, but I consider that the price of admission!