Thank you Andre. It should be noted that the full octave difference is a comparison of two extremes, normal vs random incidence.
My real point here is that we have side and vertical modes in action at the mix position. In a typical 8 foot high room the listener is at the nulls of both side and height modes. Then the combined peaks at X2 and so on.
I could make a case for asymmetrical listener position here.....LOL
It seems to me that side reflection and cloud panels should at least attempt to have a go at those modes. The extra depth of a thicker panel, an additional airpgap, FRK on the back, causes little extra expense or hassle as we are already treating HF in any case.
Ultimately the more space efficient designs seem worth consideration.
Helmholtz perf, Limp Membrane, Panel, VPR (floating steel panel), with a HF absorbent front seems to cover all bases.