Healer delima, advice needed.

Ok so I have 4 shaman and a paly. I enjoy both PvP and PvE a lot. I have just got my paly and shaman team farming about 5-7 heroic bosses a day. I'm gearing up with PvP gear nicely. Heres my pain. I had to respec one shaman to resto to be able to heal through heroics. So if i want to PvP i have to respec her. My problem is that i do both everyday.

So I was wondering if i should replace one shaman with my 70 priest. His gear is not "great" he has a mix of PvP epics and a few mooncloth epics for healing. With better enchants he will be at 1400-1500 healing. My SHamy has 1200 with enchants and consumables. I seem to always be oom after any large pull or any boss fight. I have to drink once, at least, and am oom for the last few hit points of most bosses it seems. And on big pulls Its hard to keep agro when shes spam healing me. I have no trouble keeping agro on 3-4 mobs or any boss ( My paly has 11k hps..280 spell damage and 494 def atm14k armour). Would the priest be better for my team i would lose the mana tide but gain lots of other healing pluses. Any advice?

Honestly I think it might be your Paly that is lacking, and not the heals. I would work on getting your palies gear up to par for heroics and see if that doesnt take the strain off of the healer mana.