I haven't done a group of Druids past 10 but I can tell you that from having a 70 Druid that going Balance is the way to go. Feral suffers a lot of location and other than moonfire, Starfire and Wrath have long casting times so moving around to get the feral in a good position may prove a bit hard.

Keep in mind Moonkin aura doesn't stack though unlike Totem of Wrath.

From my experience putting in a few points into Resto will makes Balance druids decent pre-outlands healers. I never had a problem healing anything up thru Slave Pens with my Balance druid.

You might consider adding a Shaman to the mix for Totem of Wrath and Reinc/OOC rez for wipe protection.

I do <3 my moonkin and I can imagine what it would be like having 5 of them call down Starfire on someone.