You should watch Toned video guides on heroics with 5 shaman. I can assure you doing a healer, tank, 3 dps bosses do not drop that fast. You have both setups so my suggestion would be to go 5 shaman and see how it goes. If you think they are lagging try the tank. When you do the tank it makes heroics slower and more of a PITA because you are tanking healing and dpsing at one time. Its much easier to just nuke.