I saw this sub at Elemental Designs.
http://www.edesignaudio.com/edv2/product_info.php?cPath=92&products_id=448

Here are the specs.
Power Handing : 100w
Available Voice Coils : Single 2 & Single 4
Sealed Box Size : .75-1.00cF
Vented Box Size : 1.25-1.65cF
Qts: .48
Qes: .64
Qms: 7.50
Fs: 32
Re: 4.1
Vas: 40 L
Bl: 6.8 T*m
SPL: 84.95 dB
Sd: 220 cm^2
Xmax: 6.00 mm
Voice Coil : 25 mm

Do you think it would be good for the WO? They are only $15 each. I already have the mdf and a 75x2 amp so this might be a cheap build.

Efficient?  Uhm I thought Efficient was somewhere about 90 db sensitivity...

In any case nice cheap woofers... You could prolly gain a bit by getting the single 2 ohm coils, and getting 4.  Then make 2 WOs or create single WO with a V shaped baffle.

That alone jumps the sensitivity up 3db, plus its the same resistance....

More displacement the better!

and as far as subs go, i wouldn't pay too much attention to sensitivity.