I'll check later this week when my new scope gets here to be sure, but if I recall correctly the .30 slug liner made an improvement on slugs and didn't change the pellet performance at long ranges.
Did your pellet groups open up with the same tume? I know it is shooting them at slightly different velocities for nearly the same grain weight, and a decent amount slower with the additional 5 grain slugs.
Maybe a different tune is in order for the two different types of projectiles.