Because split group exp is terrible for us, i'd say stay away from mob grinding. Quests are our biggest source of exp since it doesnt split, so i'd say if anyone really cares about speed leveling teams, then to make a multiboxer specific questing guide similar to Joanna's, but obviously no collection quests ect, and to maybe use a set of warlock planted in specific quest hubs for faster travel. If someone made the effort to make a guide i bet a group of 5 wouldnt be so far off of soloing speed. The biggest issue would be traveling time imo.

I have a new team i'm going to level Alliance side when i'm not so busy with retail, so i think i will make time to set up a questing guide like Joanna's for multiboxers. Hopefully it works out.