Rob
If you can hit a pie plate with it off hand at 100 yards, I would leave it as is. I would shoot it at 200 yards and see where the right and left bbls hit.

What load shot the 2.8" six shot group? That sounds pretty good at 100 for a double rifle to me.
Also you could adjust your scope so that the rounds from the left bbl hit 2.5" to the right thus the rounds from the right bbl would hit 2.5" left. This in effect would "center" your group to the crosshairs. This way the shots from either bbl would only be off 2.5" at 100 yards.

Still, I would like to see the 200 yards results before re-regulating.