The front corner of the forcing cone is where the pressures are highest. .090" is considered the minimum safe thickness at that spot.
Lengthening a 2 7/16" chamber to 2 3/4" (16ga.) probably took that wall thickness down. A gunsmith with a chamber reamer is only as competent as the advise he gives you.
Shooting one brand, which gives you some issues, in opening the gun can be the result of a lot of things, the least of which is the 2 3/4" shell.