Do you Horde Warlocks find yourself threat capped at all during raids? Or is this something that you don't really run in to until later tiers, when you start getting better itemization?
Reason I'm asking is I'm considering a MD/Ruin spec. I thought Improved Imp would be nice for the tank group and the 20% threat reduction might sometimes provide a dps boost by raising the threat ceiling. I'm not sure if it would be worth it over an SM/Ruin spec or 27/3/21 spec though, with the big thing being Imp. Lifetap