My priest/lock team is 67 and now and they are Afflict/Felguard and they put out just a SICK amount of damage. With 16 debuffs on the target (all affliction mind you) their drain life ticks for about 325 when the target is loaded with DOTS and Fel Armor is on. By the time the target is out of the consecutive stuns from the felguards they're pretty much dead.

One of the nice things about multiple warlocks is that if you spec your priest to spirit of redemption you have 12 seconds to heal your locks for free without interruption and then just soulstone back. You have 4 soulstones to play with in 30 minutes and that should give you some staying power. If one lock dies, res or summon them back. provided you have the time to do it.