Can't just go with the surface area. You need to calculate the Vd...and I'm betting the captivator driver will have significantly more displacement than these pair of twelves, but that's just me guessing. I'd love to see an audioholics review of the captivator, captivator s, s2, and/or any other JTR offerings, as well as a review of Seaton's offerings. I'd guess a submersive F2 would be easier to review than the normal submersive since both drivers are on one side.
Anyway, I am of course eager to see the RBH sub review, even though units like this are well out of my keyboard commando price range