The haste increase is equivalent to 38.06% (400/15.76 = 25.38% assuming 15.76 haste per % decrease) speed at level 70.
This axe's proc is not normalized to one proc per minute (1ppm). It appears to be random and depends only upon how many hits you can land. This ability may not proc for minutes at a time and then proc on many consecutive hits. The haste does not stack, but the timer will reset.
In theory, this axe might scale well with other haste items, as the proc is not bound to 1ppm. Achieving a consistent haste buff may make this weapon proc even more, causing even more procs. Items like , , Thundering Skyfire Diamond, and other haste buffs could, in thoery, greatly increase this proc.