I just recently got an endorsement deal with BKP and I'm madly in love with the Nailbomb! I've seriously never heard a better pickup and Tim is one of the coolest guys ever.
In my RG guitars that I've ordered Bare Knuckles for, I've gotten the short-leg variety and it seems to be the way to go, though as Albee said it won't matter all that much outside of a direct-mount situation.
If you're looking for an EMG 81 sound, try a set of Bare Knuckle Miracle Man pickups; from what I understand, they were designed to deliver an 81 tone without the muck and razors of active pickups. The Warpigs are awesome too, and will give you a more useful
clean tone overall but have a different vibe than the MM set. Somewhat darker, I'd say.