The SX3 is definitely capable of putting up some big numbers with its backbored barrel and invector plus choke system.
Were you shooting at a large piece of paper to make sure that the gun is shooting to point of aim?
Have you tried to adjust the stock with the drop and cast shims?
What kind of shells were you using?
Short of Nitro's, I have had my best patterns with an Indian Creek .675 shooting Hevi-13 3.5" 2.25oz #7's. Having said that Hevi-13 3.5" 2.25oz #6's, Hevi-13 3" 2oz #7's and #6's all performed very well too.