A 4" mid crossed high order at 1.8khz will not be meaningfully narrow. At some point you gotta drop the dogma and focus on actual performance... which you're not actually focusing on.
The Accuton C90 is bigger than you think it is. Sd is ~80cm^2.
A 4" driver (e.g. the ScanSpeak Disco 10F or Audax HM100Z0) will be less than half of that.
Though I did think the crossover was higher than it is (I'll assume you're right), there will still not be what I'd consider an acceptable directivity match under those constraints. Certainly not one like to a concident/Dual Concentric driver with proper crossover, or a tweeter firing through a waveguide the same size as or slightly larger than the driver below it.
So despite Jim Salk's obvious woodworking chops and Dr. Murphy's crossover design skills and voicing talent, I still say that such a speaker is not of a kind with consistent-directivity narrow pattern speakers (GedLee, Tannoy, Genelec, JBL Synthesis, KEF*, Pioneer Elite/TAD*) or consistent-directivity wider pattern speakers (Revel, JBL LSR32/LSR6332).
*The KEF Uni-Q and progeny (Pioneer/TAD) tend to converge on a 90deg pattern, but lose pattern control higher in frequency than speakers with larger mids. Except for the smaller ones, which seem to have wider patterns.
How would you compare the Accuton vs the BG Neo8 midrange?
I wouldn't, because I haven't heard the Neo8. The only small planar mids I've heard are the Eminent Technology panels they sold (maybe still do) for car audio, the old Monsoon computer speaker panels that were essentially mass-produced variants of the ET panels, and the ~15 year old BG "Acculine" series (also marketed primarily to car-fi). Honestly, that kind of driver doesn't interest me much. If I wanted to deal with the placement restrictions of a directional dipole planar, I'd just buy Quad ESL-63s on the second-hand market and set them on DIY'ed dipole bass-bin stands.
FWIW, if I had to use a 5" midrange, the Accuton wouldn't be in the running. I'd probably be looking for a properly-stored NOS stash of Audax HM130Z0's, but assuming those weren't to be found, my first calls would be to B&C and BMS. Or, I'd comb Florida for speaker reconing shops that carry a recent-model 5" Uni-Q.
It occurs to me that another speaker you'd probably like quite a bit is a the Gradient Revolution. While its Seas coax isn't on the same level as modern Uni-Q's (Seas licenses IP from KEF, but IMO their coincidents are still at about the 4th gen Uni-Q level), the crossover is extraordinarily well-designed and the mid-to-upper bass is among the best I've heard in a commercial loudspeaker. Dunno how available they are here, or if they'd work as well in a typical U.S. cardboard-n-spit room as they do in a European masonry room.
