I have two 12 gauge 1100's that I have modified by cutting the barrels to 21 inches, adding high vis sights, and magazine extensions to. I did one in the early 1990's and one about 10 years ago. It is relatively easy and cheap to do if you are handy with basic tools and can follow written instructions. The hardest part is reblueing the cut point on the barrel and adding sights. You could just buy a 21 inch barrel with sights, but you could pick up another 1100 at a pawn shop for what that would cost you.
In any case, both of mine function fine with buckshot, birdshot, and slugs. Slugs group 8 inches at 100 yds, while buckshot opens up to about 15 inches at 25 yards, so that is what I consider the maximun ranges for those loadings. I use the birdshot for skeet and balloons to stay in practice.
The handling characteristics are a lot different with an extended mag that is fully loaded, as the gu is not just heavier, but more of the weight is forward.
You will never begin in the fight you have planned for. You will begin in the fight the other guy has planned for. Retreat in a manner that leads him into your fight should he press his attack.