You heard it when you demoed the S2di, I remember you being quite impressed with this woofer.
Longer excursion combined with a significantly more powerful motor. Also having much lower inductance greatly improves transient accuracy so the woofer has more punch.
Yep, that was scary... SEAS has since built quite a few more for me since that time though.
Efficiency is basically the same as the S2 woofer (a few tenths of a dB less) so using this woofer would not increase efficiency. As far as a rough cost, impossible to determine just yet. I am looking into large volume purchases to see what the absolute lowest pricing could be. This is really a $300+ woofer if mass produced. It is exclusive to us and there have only been 8 made so determining pricing is a bit complicated since there has never been a production run on these. All 8 have been hand made samples.