According to tests with Beast Lore in
Curse of Weakness you only really benefit from Improved DS in BC if you also apply Curse of Recklessness. Tested on, as I recall, Outland beasts ranging from raptors in Blade's Edge, through Uvuros in Shadowmoon and finally the Lurker Below in Serpentshrine; all these were capped at around 315 AP reduction. The working theory was that this is simply because mobs don't have any more than 315 AP to reduce.
It's also possible that non-beasts have more AP than beasts.