Forget what combos everyone else tells you. Each gun has its own likes and dislikes. There is no way to get around testing your gun.
Example, this year the "hot" slug has been the new Hastings. I bought a 20 ga slug barrel for my Encore, bought 3 boxes of the Hastings and went to the range.
There I shot a nice 3 shot 10" group at 100. Needless to say did not shoot the other two boxes. However, there was a guy with 4 shotguns, 2 12 ga, and 2 20 ga siting them in with about $600 of slugs of every brand.
I swapped him two boxes of the the Hasting for two boxes of his. One of them was the Win PLat supreme, 2 3/4" 260 grains. I shot them first, got a 1 1/4" 3 shot group, adjusted zero with the other two and immediately went and bought 5 boxes. Two days later on opening day, dropped a nice 17" 9 point at 168 yards, which is very decent for an east coast VA deer.
Now if you want to make a dedicated slug gun with the 870 or 1100, have your smith put 1-2 of the 8x40 screws in the side of the action into the barrel to lock it up in your action and then loctite it when done. That alone will cut your groups by 50%.