I really like a Discipline Priest.
You're still healer focused, with few/no points into Shadow.
You lose a bit of AoE healing power, but gain a lot of mitigation.
Still have the same buffs too.
This is quite possibly the strongest single target healer in the game.

Not sure if that's optimal for boxing.
You need to combine Power Word: Shield and Pain Suppression into macro's.
Heals will likely be the same, except Penance is virtually mana free and a very strong heal.

You'd need a second healer, preferably a different loot table, for raid healing.
A Resto shammy is extremely good at raid heals, but a Holy Word is exceptional when toons are bunched.
Not sure if the Resto's Chain Heal is enough in this regards, but Earth Shield is very powerful too.