I find this interesting as the baffle in the "soffit mount" is what gives you the 6db of gain.
I should add "in the bass region" which is where it is nearly impossible to keep distortion low at high SPL's.
You get even more bass SPL gain by placing a speaker tightly in a corner but this exacerbates room modal issues severely and is detrimental to mid range and treble as you get early side wall reflections.
This is the same property as a horn with all of its issues.
Well sort of...a horn takes this to extreme by coupling the transducer to a very narrow/small chamber(s) and this can cause non-linear distortion due to heavy compression of the air within this small space (when played at higher levels). Technically short wide open horn shapes (like you see on many tweeters) are called "waveguides" and they do not cause distortion although they can alter dispersion in a desirable manner.
FWIW: horns have one huge advantage over conventional designs in addition to their efficiency => lack of compression. Generally a horn will preserve the dynamics of the music at high levels much better than a conventional transducer. In conventional designs the voice coil gets very hot (thermal compression) and large excursions mean non-linearities (distortion). Horns are an excellent approach to many high end applications as they do have distinct advantages.