If you can't get the pattern to come down any more then try a different shell and load and/ or a different choke. The other gunsmith I work with just this past week went out and patterned his 870. He used 4 different loads and three different chokes and all of them impacted the target in different places. The tightest and most centered pattern was with Remington 4X6, three inch shells and a Briley straight rifled tube.

If none of that works you can have your barrel bent to adjust you point of impact.

There is no one choke that is going to work best in every 835, 870, Benelli SBE, etc... You just gotta try one out and see what happens.