I have a hammer Remington pigeon gun which started life as a 10 and was chamber sleeved to 12. I bought the gun as a sleeved chamber gun and it produces great patterns. I also have a single barrel for an 870 Rem made by Moneymaker Guncraft that is 10 with a 12 chamber, again wonderful patterns. The only issue I have heard of is in cold weather where the plastic skirts on plastic shot cups may not flair properly and allow pressure leakage.
I don't shoot either of these guns in COLD weather so I haven't experienced this personally.
