I don't really see the need to go to a different slug unless you just want to.......with the accuracy your getting with the winchesters, and the price, unless you find something cheaper, why? Your already shooting much better than I would have expected.
I've chased the same hole for many years, and spent loads of money doing it.........but for deer at close range, I don't need to eek out the last bit of accuracy.......Now if it were a heavy varmit, benchrest, or one of the latest wiz-bangs, and I intended to shoot small targets at extended range, then I would probably experiment..........after all, how many slug rounds per year do you think you'll shoot out of that gun? If it were me, it would probably get zeroed before the season.......shot a few times to get familiar with the gun......used during the hunt.......cleaned and back into the safe.......or the barrel changed and used for small game........
I'd be happy is shot so well on the cheaps!