That's exactly what I am saying. Why would a SPL purpose driver be inefficient? That makes no sense at all. Efficiency is power out for power in.
Why would it be built with a heavy cone? You want as light, and stiff as a cone as possible. High BL^2/Re and low Mms are your main priorities in designing an SPL driver. To handle an absurd amount of power, yes you probably will have multiple stiff spiders to control it, but this will not effect efficiency, only frequency response.
If I need to explain the aforementioned sentences then I would suggest you do a little bit more reading yourself //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif