Got it!

been asking all my old rc plane mates about this, and Ive found my 300psi figure.

Apparently, the old cylindrical 3 litre coke bottles were ok to 300psi. This was not the case for the smaller bottles, infact, the sculped 1 litre version blew up at half this according to the bloke I was talking to.

So how does he know? he tested them a few years ago, whacked them on a standard test rib and noted them all down.
He currenly uses a 3 litre coke bottle at 300psi in his quad pulsejet powered Vulcan.

there we go, I knew Id heard 300 somewhere.
as for the rest, Id reccommend we look at the coke bottle issue, with possibly more tests, as 50psi was there for rex, whats the difference with 70psi, or 80? if it saves money and is safe why no go for it?