I know about the tree, but I'm wondering about gear that I can wear. I know of one piece that increases immolate cast speed but that it.
As for the dps, that's really not what I was going for with the imp.
However, when fighting 70+ elites, he'll live much longer than the fel because he won't get aggro like the fel does. At 70+, elite bosses hit the fel too quickly to heal him, in my experience. The imp might only do 300~400 dmg per shot, but he'll never get attacked unless it's aoe or no crowd control.
I like to think of the imp as another DoT.