An interesting alternative. My four 'locks (lvl66) are affliction specced atm, but I guess I'll try demonology now. Sounds like fun.
If anyone does prot paladin/four metamorphed 'lock instance runs, a little video would be awesome.

Update: I have respecced. It's absolutely amazing: 4x66 locks and one T4-Tankadin currently doing slavepens.

Oh
my
goodness.

What a bomb fest!

Feels like 1000 dps per warlock to EACH TARGET in range.
It's now a matter of: How many mobs can my pally pull together.

Update2: Underbog is great! Right at the beginning of the instance you can do a couple of large pulls.