That makes sense, because given the pressure regime inside the silencer, the gas flow split between drain hole and tail pipe will be in the ratio of their areas. A 1/16" hole has a CSA of ~1/256 sq.ins., the tailpipe is about 700 times bigger. Just over 0.1% of the exhaust gas gets out of the drain hole, and for all practical purposes the hole might as well not exist. If it worries you, plug it with a self-tapper. But if you don't, it'll make no never mind, and your silencer, given that you live in cold, damp Sydney, will last forever. Personally I'd make my decision on whether or not it popped on the over-run. If it did, I'd plug it. Scares the horses otherwise, and attracts undesirable elements. Like Plod. And since you aren't allowed guns now, they'd probably mistake it for gunfire and shoot you. Or is that the Met I'm thinking of?
Tom