I was under the impression that Doomguard didn't scale with haste at all. However, after several tests with IOTC, his cast time is reduced when I use him with the proc active. Did something change and I missed it? I don't remember this being the case at all. I was under the impression it only scaled with Demo mastery, SP, and Crit.
Edit: Just noticed that no matter the cast time, it seems really inconsistent and there are pauses between the next cast the lower the cast time gets. Guess that's why it's never mentioned to scale with haste. Should've just looked up information on it more. w/e =p