Andy - a couple of points in a very complicated maze. Choice of impedance indirectly includes slopes because slow slopes require less moving mass to avoid self-attenuation on the treble roll-out, and lower inductance for the same reason, and therefore fewer coil winds.
Note that the 1.5 drivers were entirely developed by Thiel in-house; they include under-hung coils, copper shunts, focusing pole and plate geometries, etc. Thiel had a complete driver development lab that was fairly robust by the late 1980s. We developed our drivers and turned the designs over to Vifa for production engineering and supply. We were too small to "own" those designs, so we allowed Vifa to put them in their catalog to recoup their tooling expenses. So, they could be considered "off the shelf", but they got on the shelf by our putting them there.
I have an eccentric theory about why phase-coherent x time coincident speakers are "harder" to drive. I believe that coherence allows our auditory perception to be accepted as "real" and not "reporduced". We take that "real" input much more seriously, and many aspects become sonically important, whereas they were not even heard in the incoherent speaker. I have previously described how I came to form this theory - it is an important leg of my sonic understanding stool. That phase-related amp misbehavior from the amp driving the Thiel low impedance load is obvious, but if driving a high-order low impedance load, I believe it might not be obvious.
