If there is a high level character in a party and nearby, your low level characters will get significantly reduced experience from every kill even if that high level doesn't even enter combat.


What a high level character can do well is do pulls in instances and leave the party while your AOE group mows everything down. For instance a level 60 priest can pull SM armory in 4 pulls and cathedral in 6 or 7. Of course your other 4 character will need to be all casters to kill all the mobs before your puller gets teleported out.


Some experience numbers from my group. Level 60 priest pulling and playing an alt mage. Me playing 2 mages and another guy playing a warlock.
Deadmines: 25k exp/h without the warlock. Priest was doing damage on most pulls with elites.
Stockades: 48k exp/h. Cleared the whole instance while hitting instance cap so can't go faster than this.
SM: 30k exp/h at level 30 with just graveyard and a few pulls in library
SM: 80-90k exp/h at higher levels with clearing Armory and Cathedral but skipping bosses.
ZF: 95k exp/h doing scarabs, zombies and pyramid trash mobs.
BRD: tbd. Torch room farming sounds most efficient but we'll find out when we get there.