Mabat
Thanks for those plots. They prove what I have said from the beginning: that a waveguide does not have the polar limitations of the source. This was predicted by the theory, shown by my measurements and now again shown by simulations. I hope this puts this incorrect belief to bed, Finally!
The waveguide does narrow as the angle is made wider, but this is not driver dependent at all. In reality driver wavefront coherence degrades at the upper frequencies and that causes reality to differ from the theory, but this is due to the inability of specific drivers to maintain a flat wavefront. It is not inherent in the design itself.
Improved phase plugs may help this situation, but little to nothing is known about the flatness of driver exit waveforms. About a decade ago I showed (B&C) how this situation could be measured in a PWT, but it was never followed through. To me, that would be the next piece of the puzzle that will need to be analyzed in order to progress the directivity control to ever higher frequencies - if that indeed is actually worth doing.
I have my doubts.