I tried replacing my alliance demon spec warlock with one of my shaman for a test. The warlock has several PVP gear items and has a +1000 spell dmg while my shaman only have a +550 or so.
I found all of the warlock spells cast really slow, SB takes 3 seconds, incinerate takes 2 or so. the DoTs are instance cast but require 15+ seconds of life span to be worth anything.
I didn't try using seed of corruption. I didn't try life drain or anything like that either.

long story short, the warlock had the lowest dmg on the team, close to the shaman that was assigned part time healing duty.
if I tried to use DoTs the shaman would burn the guy down before the DoTs did anything.
if I tried SB the shaman did 2x the hits eating up any +sell dmg the warlock had over the shaman, the shaman had twice the chance to get a crit in and have a 40-45% crit.
incinerate has a 2 sec cast and the shaman still out produced him.


The warlock is better for the long fight where the DoTs have time to ripen, I was farming Mech for totem of the void and the warlock wasn't much help.
I may try it one more time with DoT + life drain and see if performs any better. I may reset recount before a boss fight and see how he does.

btw: got 4x totem of void now, just one more to go!