This question has "haunted" me too.
What I think you need to do, is go test your DPS vs 1 mobs at a time, find out how good you dps the mobs in various zones, and how many HP's they have. While you are at it, also note how much XP they give.
Next you should concider if you can increase your DPS by fighting multiple mobs at once, this will increase damage from DS, Riposte's and AE's, but remember that Aux'ing works for mobs too...
But again, test how many you can take down at once, and how great your DPS is then, ect ect.
Finally you have data to make an Exp/Time ratio.
Oh, remember to come post your findings please