Hey, I can actually give you some input on this as I tried it.
I ran (in maraudon):
50 warrior
50 Mage
50 Mage
50 Lock
60 Priest (in 4/8 T2 with +600 healing)
Honestly I stopped it, runs went more smoothly with a level 50 paladin than a 60 Priest for me.
The main problem is threat, the 60 priest pulls crazy threat of the tank. I struggle even building up enough threat on the mobs because you have to heal to keep him alive thus pulling with the healer. On single target mobs it works ok, but once you have a group your healer is tanking
There is probably ways to make this work though, just suggestions of the top of my head:
-In lower level dungeons you can just tank with your cloth/leather healer
-If you run without a tank and just 4x mages, warlocks or hunters or something
-You have 5/8 T2 with a priest for automatic fade
-Your healing with a Pala/shaman with a shield and decent defensive gear, and they cant be interrupted
-Druid Tank might work better
Edit: Just want to add that I didnt overheal, I'm specced for threat reduce on the priest and spammed fade on cooldown