I run this setup aswell.

4x shadowpriest + VE is enough to keep your tank up pre-60 instances, imo. Toss in a couple of renews now & then (but say goodbye to shadowform) and you'll have no trouble at all. In Outlands (I have only done Ramparts)
this didn't really work. I specced 1 priest to Holy as the main healer and everything works great now. I feel like I have somewhat slow dps though, this due to the fact that Mindflay cannot crit, I don't know..

Vampiric Touch will be a disappointment when you hit 50, but it gets better in Outland as you get more spelldamage.