So Paladin, Priest, Druid, Mage, Shaman..

Idea 1
Druid (tank)
Paladin (healer)
Priest (shadow)
Shaman (elemental)
Mage (fire)

Comments: Druids make good tanks for the time being and switching back and forth between bear and kitty could make world questing easier. Also paladins are one button wonders when it comes to healing meaning that you simply have to target manage and spam a single spell. Also paladin mana seems infinite this way. Priest will have passive mana and health regen for party as shadow and the spells are relatively easy to castsequence. I'm not personally familiar with Shamans or Mages but I imagine they can be castsequenced also.

Idea 2
Paladin (tank)
Druid (resto)
Priest (shadow)
Shaman (elemental)
Mage (fire)

Comments: Only change here is tank and healer. I like druid heals more, but mostly late game. Rolling hots are more my style than paladin healing. Especially since they keep working even after you change targets which can be nice in certain situations.

Your current setup is probably just fine. I've never boxed a priest healer before, though I have played a holy priest a ton and I'd say that they are not the simplest class to heal with. They have a lot of healing utility spread across a number of abilities, many of which take some intelligent downranking for good efficiency and you might find yourself having trouble micro managing this while boxing 5 toons.

How are you doing target management? Are you using a focus setup or sticking with a largely focus free setup?