My 5 man group make up is close to what you are thinking about. Just I use a shadow priest instead of a lock. The stam and spirit buff+fear ward are nice but a lock wouldn't be a bad substitute. You could use imp for a hp buff and ss the tank just in case you get a quick whacking and try to save from a wipe. I really like shaman healers, sometimes earthshield is all that's needed to keep the tank up and it frees up some concentration which is needed for a 5 class group.

If you do go with 2 boxing it might be a good idea to go with a tank+healer setup that way when you are 80 it'll be easy just to get 3 dps pugged. Everyone is always looking for a tank and healer at least on my realm.