With the right ammo and barrel combinationi would think a 3 inch group would be about right at 100 yards. The Vital area is how big at 100 yards. Just remember we are shooting 100-150 yards or so max so minute of angle is not needed or required.
I'm sorry, but I would expect better. I get 3" groups out of a 12ga Ithaca with a smoothbore barrel, and we all know that the 20ga performs better than the 12ga. I would expect something in the 1"-2" range at 100 yards
That is pretty good grouping for a smoothbore and probably a result of the barrel being screwed to the receiver as well as a slug that fits the bore nicely. I don't know how the 20 would be better except maybe in the recoil department. Don't know much about 20 gauge slug guns could you elaborate ?