I'm not sure about a scuba tank, but i had the high pressure line (between tank and first-stage reg) on a Scott 4.5 SCBA blow on me at 4500 PSI a month ago, sounded like a gunshot going off. and the tank started to drain pretty fast. It happened right as we opened the tank for the weekly check.
The tube to the SPG, while at full pressure, is low-flow, so you won't lose much air, since it's static pressure. We test fire hose the same way, through a valve with a 1/4" hole in it. fine for static pressure, but if you have a blowout, the flow will be minimal.
The odds of a blowout are less at depth than at the surface, due to the lower differential between internal and external pressure, probably not MUCH lower, since we're only talking a few atmospheres atr most, but there it is.