The dreaded flat spot has so many theories behind it that I'm starting to think J. Edgar Hoover is alive and involved.
Do you have the mixture plot for the dyno runs you've listed above?
I'm starting to play with a new-concept replacement for the emulsion tube produced by a fellow name Keith Franck. I installed them today, and have a sense they've made a difference. https://www.webstore.com/82598,owner_id,other_items
First, my car has the OKP bespoke manifold. I believe a desiamesed manifold is still going to be a compromise as compared to a correct, straight-shot Weber manifold. Turbulence can do nasty things.
Keith's work appears to be addressing the richness that often occurs during WOT in the middle RPM range. I'm including the last dyno run I did on my 2300, and you'll note that, although the torque curve is fairly flat, the mixture goes off-the-scale-rich during transition. Presumably, this can be fixed by juggling alternative E-tubes, mains, and air correctors. Before I did that, I tried Keith's "VF" tubes. I think it's working based upon the Mk1 Rumpometer.
I don't know what he does for Dellortos, but maybe he has something for them as well. I'd try a set before hanging plumbing all over creation.
I think it's meaningful that the Cast Iron 2000's have always been fussy to convert to Webers, even when de-siamesed. With my OKP manifold (and the essentially identical FNM manifold), it was very nearly plug and play. In all cases, I've plumbed the float bowl vents straight into the air plenum, sucking on the same air adjacent to the carb air intakes.
The red traces are with FNM manfold, 40DCOMs, and stock cams. The Blue curves are OKP mani, 45DCOE9s, and improved Webcam cams. (Idle's great at 700 RPM)
On the red runs, we shut down at 6,000 RPM, as indicated on the dyno screen. I was handling the throttle on the blue runs, and shut down when the tach said 6,000, revealing about a 10% difference between a calibrated dyno, and Italian tachometers. I plan to revisit the dyno at some point this summer, and compare the VFs, etc, plus go to the same 6,000 calibrated revs.