You need to know where the densest part of your pattern is so that you can make adjustments to bring the densest part to the point where you are aiming. This can be done using a scope, adjustable iron sights, bending the barrel, or the cheapest method, Kentucky Windage.
The choke/ shell combo may not be compatible. I have seen that very combo shoot #5 lead that well at 30.
A #6 or #7 Hevi 13 may do better or it may take a Rem Super Full choke, Carlson or Trulock 570-575 ,etc. and of course the Truglo SSX 550 if you are not into custom chokes.
Since you are talking to William at Sumtoy he can get you exactly where you want to be and work with you. The extra time spent with him will be worth the money as well to get a super choke that will be there as long as the gun itself. If you aren't happy he wants to make you as satisfied as possible.