Great review and comparison with the JB on the SD blog:
Review: Seymour Duncan Custom Shop RTM Humbucker | Seymour Duncan Blog
It seems to have a slightly more beefy bottom end, while taming some of the upper-midrange "hash" you can hear in the JB. Really sounds quite good. I wouldn't have expected an Alnico 2 pickup to sound like that, but i guess they overwind the coils a little bit from JB spec to compensate for the softer tone the A2 magnet would give.