well from a pve perspective.... hands down the best DPS class is a destruction warlock, 0/21/40.

Druid will work good for a single target tank; palladin works better for group tanking and less micro-managing.

S.Priest will not cut it for healing any heroics. Period. Respec him Holy for PoMending fun (tank to life-tap to tank). Alternatively, you could use the S. Priest to feed mana to your locks, marginally increase healing (enough to support a life-tap every now and then) and increase the locks damage I believe.

You already have a decently geared Tank.. which makes it hard to justify rolling a new tank. Although for a boxer, a pally is a no-brainer IMHO.
Anyhow, I suggest warlock x 3, convert S.Priest to healer or warlock x 2, and roll another healer.