I always associated power leveling with getting a group together and just going out and beating things in the face. But yeah, in a group you'll more than likely fight mobs that are around 5-10 levels above you (the chain xp you get for stuff like that is great). From what I've seen so far, XP group shouts have been random of late and always involve higher level players; but I'm sure that I've just missed the lower level group shouts since I'm barely in a city for too long.