Slugs, especially Foster type slugs, lose velocity rapidly compared to centerfire rifle bullets due to diameter and their hollow construction, but that is still quite a chunk of lead to get smacked with! I have fired 12ga. slugs many times on an 18x24 steel target at 200yd. and made hits even with a bead sight once I knew where the gun printed. OK Highway Patrol used to require qualification at 100yd. and familarization firing at 200yd. if I am not mistaken. 100yd. is a fairly easy offhand shot if you know your gun and ammo...as mentioned 4-6in. groups seem to be the norm in smoothbore guns.
There are many stories of Cape Buffalo and even Elephant being killed with 12ga. slugs in Africa...I cannot testify to their truthfullness, however. Myself, I think I would prefer the warm, fuzzy feel of a big bore .375 H&H and up rifle to any of my shotguns in the African bush.