Just happend to have a GM .54 in 15/16 stripped of sights and rail (prep for browning) handy, so I mesasured it for ya.
sight and rail hole depth = .140
sidewall thickness @ groove=
.190 so that's only .050
I'm not even gonna second guess Green Moutain, these are pro's that have been making barrels for a LONG time. I am confident that's with in the saftey range for pressure testing, otherwise they'd do something different.
Good question though, saftey is always a good thing too know and think about. I think the biggest deal is the small size of the hole and the screws fitting in snug to maintain the integrity of the barrel (aka: the hole is plugged). The big hole in the end, in your case .450 or better, is relieving the pressure fast enough so the small drilled/plugged holes don't have much to deal with. When ya think further, dovetail sights remove a large piece of the barrel. One I have here on an old cva barrel is .099 deep and .340 wide!
I guess I'd stay away from the all to common rookie move;; "I always double the charge, so I get a good hit!" gadd's,, that drives me nut's,, even the "I gotta know the MAX load for this gun", stuff leaves me shaking my head