Your assumptions might be right, that the inefficiency is compensated by lighter weight, if you build it like that.
But the biggest issue I have read about is the motor mixing between upper and lower "deck" being a little PITA to get right, and if you don't it will go up/down a lot when yawing for example.
Have you considered a H6 (or H8)? Those are very attractive and seem like the ultimate photo/video platforms. Something like this:
Or why not a twelvo-copter? :P
DODEKAKOPTER from khead19 on Vimeo.