There's a current debate over on
www.doublegunshop.com about this subject. There are arguments on both sides of the issue.
The main factor, if there is one, largely depends on the forcing cone. If it is tight, it can run up pressures. And 1000 psi is a 15% pressure increase in most guns, which are rated for 7000 psi.
LOts of people on that site shoot long shells in short chambers, however, and report no problems. I won't, mainly, as someone noted, the 2.5 16s are about as cheap as 2 3/4" shells, and my 16 is 100 years old, and don't want to take a chance. Also, Michael McIntosh speaks againt it. I have, in my foolish youth, shot a 3" shell in a 2 3/4 12 ga, and could tell the difference in heavy recoil. Heavier than when shot in a 3" gun, when the shot charge met the forcing cone and compacted too quickly. Also, the shot in modern shells is somewhat compacted because of the plastic shot cup. But I suppose it would still have been excessive even with felt wads and no shot cup.
A modern enbloc shotgun will take a lot of pressure and probably won't blow up with a shotshell a bit too long, but then, it probably doesn't do it any good, either.
Here's a link, if anyone wants to read it.
http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=87561#Post87561