I don't think the developers take multiboxing into consideration at all in regards to balancing the class/spec.

Even if they did I think it would be the OPPOSITE since more than 1 dev has admitted to multiboxing (and most likely 5 shamans based on the popularity )

Anyway, we'll see how things change in the next month or so - yes our DPS is low but as far as multiboxing is concerned I'm quite happy with where we are and plan to continue with 5 shamans (and at this point probably wont use the pally for PvE at all either - will either do 5 shamans or not run the instance).