[Can you put a 2 3/4" barrel with the 2 gas ports on these and shoot lighter loads?]
Not AFAIK. I believe the Model 1100 feed mechanism for the 3" guns is different than that for the 2 3/4" guns, and the shells will not feed properly, IIRC.
If you were talking about an 11-87, then I believe there would be NP.
.