If you get felguards then you dont get Soul Siphon.. I've been playing the locks and mage combos on the ptr and the mages just do more dps faster than the locks; felguards are more of a distraction. I'm leveling locks on live servers but so far I do better with mages than with locks. If I'm zerged I just aoe the whole place.. you can do hellfire with locks but it's not quiet as effective