rfrankhauser,
If you find a combination that shoots well with iron sights, there is a good chance it will shoot well with scope( I wouldn't say this about a double rifle).Of course you can try other slugs if you want, but I doubt if any would best Brenneke. If you are not satisfied with slugs at the rifle's sight in range, try it at some other range, say 35 meters, and you might be surprised.
Mike