Well, there is some math behind it. For BM you'd be getting 10% more damage for you and 20% for your pet for 6 additional seconds. Which maps out to roughly a ~1.5% damage increase if used on CD. Using femaledwarf 320 agi is just a tad above a 1.2% damage gain.
The off tier pieces have 317 more agility and with better itemized stats. Basically trading haste/hit for mastery/crit. Femaledwarf also averages all your abilities so let's say you use your pre pot and a trinket procs at the start that would make your burst way greater than ~400 dps, but in the sim it's being averaged into all your other abilities devaluing the proc and BW. I'd say keep the 4 piece.