Last year I decided to go the 20 gauge route and bought a factory camo 870 Compact. Using a Carlson .575 and a Primos .570 I had good patterns with a variety of loads and decided Hevi13 7's were my best choice because Iowa won't let me use 9's.
Having a good supply of Hevi's that will probably be my best place to start with the SX4. I'm thinking I'll try another Carlson tube, probably the same constriction. ***I wide open to other ideas too.***
I went with the SX4 because it's basically the same shotgun as my Browning Silver and both of my sons shoot SX3's for all of their wingshooting. Pretty trouble free and very reasonably priced.