The waveguide is there for directivity, not efficiency.
Revel M105 and M106 are both rated to deliver 20Hz less extension than this speaker. Considering that, just 1dB sensitivity penalty is not too shabby!
EDIT: note that the previous JBL bookshelf horn speaker has if anything slightly lower sensitivity than this one:
It's a waveguide (device to control directivity) not a horn (acoustic amplifier). In either case, the sensitivity is going to have been set by the desired bass extension, i.e. woofer design and cabinet volume.
Sensitivity is one thing I personally have never bothered to measure, because calibrating a portable measurement system to absolute (as opposed to relative) SPL is a big pain. Also in use it has never been shown to me that important, and I've run systems varying from ~83dB/2.83V/1m to ~96dB/2.83V/1m response. As long as it can reach the levels you want with the amp you have, I call it good enough. I typically do pair a speaker with a moderate power amp (Parasound Zamp usually) and a massive amp (ATI AT4007).