I currently got a team of 4 x warlocks on level 35 and the run and dot strategy is good, but not as good as it could have been, what im saying is that when you hit level 50, respecc to demonology for the failguard and then affliction down to siphon life. It realy doesnt matter after that what specc you turn your priest into because:
a) He will be healing alot aka. shadow is not the route
b) He will be DPS'ing alot aka. shadow is the way to go, but you will be annoyed with mindflay breaking follow