Is there a real advantage for 3 1/2" chamber ?
I mostly shoot 2 3/4" and some 3" shells. The crowd I shoot with has a saying "If you can't knock it down with 3" shells a 3 1/2" won't make up for poor marksmanship or poor judgment.
The only real use I've seen for the 3 1/2" is maybe in duck and goose hunting.
My son has an 870 3 1/2" Magnum, but uses 3" for duck hunting and 2 3/4" for everything else. I don't know if he's ever used a 3 1/2" shell. Like you said earlier, it might be nice to have the option.
I agree that shooting ability can't be overcome by firepower.
I don't own any 3 1/2" chambered shotguns, and only have two guns that will use 3"..... but the same rules apply across the board.....by doing so, I would have to give up a round in capacity..... and there's not enough difference in performance (from what I've experienced in my own shooting) to warrant the use of longer shells. I'd rather have eight than seven...even if the seven are slightly more powerful. Everything has an intended use.......in hunting, power may be a slight advantage over capacity....in a SD scenario, a 2 3/4" 12ga will more than fill the requirements, so capacity is everything.