Source:
http://www.wowhead.com/news=260284/7...20-set-bonuses
Hunter T20 Beast Mastery 2P Bonus - Cobra Shot, Multi-shot, and Kill Command increase the damage bonus of Bestial Wrath by 1.5% for its remaining duration.
Hunter T20 Beast Mastery 4P Bonus - Cobra Shot, Multi-shot, and Kill Command deal 20% increased damage while Bestial Wrath is active.
The bonuses seems pretty strong but I will really miss tier 19 4 set with the BW cd reduction.
Its kind of lame that Mantle will probably pull ahead even more as the way way OP legendary since you can reset BW a lot quicker without wasted DB procs. I'm at 6 legos now, I guess in another 4-5 months I'll have the entire collection and mantle :P.