Hey Zimm. All you horse flogging has caused me to take a closer look at what I was posting. Johnk is, of course, correct (big surprise). I promise to stop making generic low = more power statements.
I have built subs for car use since I was 14 and always found that even with the best subs (presumably of acceptable efficiency) you needed substantial power to get acceptable results.
Design both of the driver and the box does have a lot to do with how efficiently the sub delivers its bass. All the really big drivers I have seen used in the DIY world seem to need a lot of power for the really low notes. I think it has to do with needing a big motor assembly to reach the high excursion needed to produce those sub 20Hz notes and still maintain linearity as the driver approaches it maximum extension. The big pro drivers seem to trade off low end extension for efficiency and headroom higher up.
Sealed designs rely on eq boost and room gain to maintain linearity. That can take a lot of power. Horn loaded designs seem to rely on gain from the box to increase efficiency.
