I leveled 3 locks with my prot pally and holy priest, and it was slow going due to the tank's slow killing, and only sped up once the locks started getting gear and lvls, then i was roflstomping stuff. I've boosted from 10-60 before and it definately slows down as the toons being boosted get closer to the booster, unless they are also capable of increasing DPS.

Just do AV, its what I plan on doing to my next 2 teams, either exclusively, or avoiding collection quests