Pretty sure Void Torrent has in most situations imaginable more burst than the DP you get from Damnation. Again, Void Torrent, even if not channeled completely, gives you a Devouring Plague on top of its own massive damage. Sure, if you get kicked right away, you wasted it, but you should prepare those situations. At cast it's 15, 1s 30, 2s 45 and 3s channel duration 60 insanity. One second channeling already gives you 30/50 insanity needed for a DP. Void Torrent deals twice the amount of DPs entire damage.
I just don't see why Damnation should be better for burst. Using Void Torrent when you're swarmed by enemies might be a bad idea, but against 1 or 2 you can take out with your CCs? Not sure.