If you haven't already, the link below contains a lot of information. Lots of measurements, but also lots of measurement issues. I think the "clean" measurements start on page 6 of the thread. Looking at the data, it seems to do a respectable job up to 16KHz. A compensation network similar to the one on your 4-PI should flatten that somewhat.

I've never built or heard this version of the smith horn, so I'm only going by feedback from the AudioHeritage guys.

Gar.

• 1200Hz Smith at AudioHeritage