Your thinking of the gimping wrong. You'll be gimped with 3 80s in your group no matter where you go. The XP allocated to each toon will be reduced by a consistent portion no mattuer what mob your killing. The total xp returned BEFORE gimping is determine by the level of hte mob.

Optimally, you should only have one 80, and your lowbies. You would be best off finding other players to fill out the group that are the same level or lower than your lowest level toon. Any instance with mobs 1-5 levels higher than your lowbies should yield the best results.