I leveled 3 Shadow Priests to 60, it was quite effective but wasn't my tastes. As for leveling 3 improved SW:P's was enough to drop any enemy my level, so my tactic was to just dot, and go to the next target. After receiving the Vampiric Embrace talent I would slap that on every enemy I could when it wasn't on cooldown making it so you didn't need to heal

With 2, SW:P by itself might not be enough unless you invest into some damage gear, I pretty much played with really bad gear to 60.