If you go with a saddle mount, then you'll always wonder if you'd have more accuracy if you'd anchored the sight to the barrel. On the other hand, if you wanted real accuracy then you wouldn't use a smooth bore 28-in barrel.
Jason, my friend, have you considered getting a slug barrel with a factory cantilever mount and keeping your 28-in vent rib barrel for birds and targets and your receiver unmarred by a saddle mount? Quite often there are slug barrel with scope package deals at significant savings.
Or, another option, how about a slug sight set that clamps to the barrel's vent rib
(click here). For the accuracy of a smooth bore, do you really need a glass sight? And, if you go with a rifled slug barrel, then you're married to expensive sabot slugs. Decisions, decisions.
Good luck,
Pete