Personally I think this is the slow way to level.

Sure all your dots probably don't finish before the mob dies but so what? Dots are the KEY to leveling super fast.

Case in point I'm running a set of level 61 shadow priest + 4 locks right now.

I literally run into a group of mobs, put 1 set of dots on 6-8 mobs, cast 2 aoe fears, and all 8 of them die before they can get back to me.

I can do this 2-3 times before having to life tap or dark pact, and my cooldowns are up on AoE fears fast enough to keep doing it forever pretty much. Sorry but theres absolutely no way a demo lock is going to take down mobs nearly as fast when you have to single or double target a couple mobs to take them down.... I can just fire and forget. Also having drain life bonuses and interruption correction helps if I need to get my life back up fast.

I really thought about going demo but its just way too fast killing things with affliction x 4 (or 3 in your case).