I've made it through a lot of stuff with 5x disc priests; They're 1x 80 and 4x 85 (full heirlooms) and have been able to clear a few heroic WotLK instances (AN, DTK, Gundrak, Old Kingdom and one other I can't recall) at 1x75+4x82ish. Having 4 or more is definitely nifty as the shared healing adds up to enough to be useful.