i would spec 2

http://www.wowhead.com/?talent=Iq0rVRckoZbxczIiz0es

and the other 2

http://www.wowhead.com/?talent=Iq0riRZbxczIizMesk

this gives you up to 11 debuffs

2 sl sl warlocks will be

corruption x2
curse of shadows
curse of tounges/curse of exhastion (depends on melee or caster)
Siph Life x2
shadow embrace (specing both with this will still only give 1 debuff it doesnt stack)
and pet of choice 2 felhounds ( i would stick dispell on auto, and silence on macro, could do both on macro but i would just leave it you have a priest main dispell, and the cooldown is just 8 seconds so 2 hounds a dispell every 4 seconds not worth macroing)

the Demon Locks

corruption x2
COA x 2
pet of choice obviously the felguards, if you leave their charge on auto its amazing how clever the ai is on this, i would have it on auto but also a macro for panic becuase this can also be used to silence really effectively if your changeing the targets.

so with this is 11 debufs boosting drain life by 44%, and draining should be very condtional as their is so much on the target he will fall pretty quickly. If its a disp priest in full s3 or somthing thats when you would need the drains, if its a fresh 70 with just blues from instances and no real pvp gear you shouldnt have a problem letting dots do thier job.

SL is very overpowered and i personally if pvp is your goal would never spec away from it. if you dont have this in arena getting focus on one of your warlocks could be too much for your priest to really heal for.