I have been long tempted to try a 5x priest group. They would have an insane instant cast full health Circle of Healing as holy. I can't imagine that it wouldn't be better to go shadow though. Now that dots will be able to crit and the fact they already tick for over 2K you might be looking at the strongest fire and forget PvP team around. Throw out vamp embrace + a dot or 2 on as many targets as you can before just spamming 5x holy nova and fear on round robin with 5x fear ward.

And instant AoE purge with 5x mass dispel if you feel comfortable using ground target AoE.

As for burst capability theres nothing that 5x mind blast followed by 5x Shadow Word: Death wouldn't kill in pvp.

Thinking about vampiric embrace though. I believe full talented it's 25% of your damage heals you and 5% heals party. That x5 could be enough to keep you going for a long time.

Providing all 5 are doing equal DPS thats about 45% of your DPS back as heals. That could easily be over 1500 HPS for each priest.

Could be a LOT of fun except for gearing em. Hope you can arena or raid because it would be very rough to do heroics with a priest tank. Even with 15% dmg reduction on shadow form and 6k + armor