with RAF I normally start boosting at 15 (as questing early levels is really quick - a couple of hours)
15-20 SFK
20-45 SM Armory and Cath
45-60 Scholomance

I'm currently running 2x Shaman (on RAF) being boosted by an 80 Feral Druid (swipe spam FTW)

11 hours, 6 mins played, just hit 40.

for reference: when i started getting boosted in SFK , i was getting 75xp per kill (obviously this gets far better when you get to the optimal level for the instance)