Hi Cal,
Yes, the efficiency of Kevin's woofer is quite a bit lower than that of the Selenium. Nevertheless, I expect that the efficiency will be somewhat better if four of them are used together in a 2x2 matrix close to the floor. By the way, I only used 6dB bass boost at around 30Hz with the OB system with four 18" woofers per side, and those woofers were in a verticle line and had a Qts around only 0.4.
I suggested that J&G check out Kevin's woofer because I myself would love to have OB bass extension down to 20Hz and thought everyone elso would feel the same way.

If I am going to build a serious OB system using drivers like the TT12", I'd want the system to have impressive bass extension by itself, and 20Hz sounds like a good number to me. Bragging right is important, you know.
Kevin's woofer does in many ways resemble the Dayton IB woofer, which is the one I would have chosen to use in my own "ultimate" OB system, until I saw Kevin's post about his new OB woofer earlier this week. I guess I am saying that I don't see the distinction between "IB" and "OB" woofers. Brute force EQ and a huge reserve of SS power should be able to solve everything, at least in the bass!

A couple years ago I helped a friend build his OB speakers using one Dayton IB woofer per side (plus EQ), and I was quite impressed by how deep and solid the bass sounded. Using four such drivers per side should be quite nice.
Cheers,
Kurt