For some reason, as to which one of you will no doubt enlighten me, they sound even better when the 6.5" are wired out of phase with the 8" & 4".If you've copied an original crossover design then that driver would have been reverse polarity as well. Capacitors and Inductors alter the phase of the signal so it's not unusual to allow the signal for one driver to go 180˚ out and correct it at the driver.
My only answer to your original question is that air doesn't flow through a driver so you can't apply the physics of a wing to a loudspeaker diaphragm. It needs to be stiff and light which is why a cone is a good choice of shape for a woofer or mid with a conventional voice coil. Electrostatic speakers are able to have flat diaphragms as the electromotive force is spread across the whole surface area.
If you don't already own a copy The Loudspeaker Design Cookbook by Vance Dickinson is a good primer for the subject... more practical than theory but a good jumping in point. If you really want to get into the mechanics of sound diffraction then the Master Handbook of Acoustics is as good a place as any to start.