I have been firing 1,1/4 OZ Lightfield Hybrid sabot EXP slugs from an Ithaca Deerslayer II. (20" rifled barrel) In inches, what is the common difference between groups fired with a clean barrell vs. a dirty barrel? (with appx 15 slugs already through it)

I know that rifles have a slight difference in groups, so does a slug barrel show more of a difference?

Situation:

I grouped the Ithaca with a dirty barrel.  I cleaned the barrel and fired a fouling shot.  The shot went 5" high , but still zeroed on windage.  Is this the normal?