Rob, my first classic double was a pre-war Sauer 16. Original short chambers. Had them lengthened, hate to think about how many 1 1/8 oz pheasant loads I pushed through that old gun. Didn't hurt it a bit . . . but it's one of those case by case deals. I would not worry too much about lengthening chambers on most older American guns, as Jerry suggests, because they tend to have thicker barrel walls than Brit/European guns. Probably the best thing on a gun with punched chambers is to have wall thickness measurements taken at the end of the chamber. The acceptable minimum I've heard, at that point, is .100. I'd have a fair degree of confidence if there's that much steel left.
Nial, per the Birmingham Proof House, the difference between SAAMI service pressure (11,500 psi for a 12ga) is less than 800 psi higher than CIP service pressure (10,730 psi). The Brit "bars" figures are crusher bars, not transducer bars. Converting to transducer bars--again, per the Birmingham Proof House--CIP service pressure is 740 bars rather than 650. Essentially, what that means is that current standard CIP service pressure is very close to American service pressure on our own old, short chambered guns--prior to the advent of SAAMI and the conversion to standard 2 3/4" chambers.
