I'll have to look into that. My presumption is the driver acts like a piston, and since the frequencies being recreated are so low, diffraction from the edge of the maniforld (and thus off axis response) isn't affected. If the opening of the manifold is less than the Sd, then the manifold acts like a bandpass filter thus altering the frequency response and requiring additional EQ.
As for the purpose of a manifold vs an array, the manifold has the woofers firing directly towards each other, thus the mechanical forces involved with their movement offset each other and greatly reduce the stress on the wall/floor/ceiling. The sides are also much smaller and properly braced. An array doesn't have that, thus it is very easy for the wall to flex/move when drivers move, which can have a dramaticly negative effect on response. Think of the amount of bracing a boxed subwoofer has. Then imagine two or four 18" drivers along one side of the box and not putting any internal bracing in it. Having a manifold also decreases the cutout into the room and allows the drivers to be vertically aligned if attached to the ceiling or floor (which prolongs the driver's length of life). There are a few drawbacks to using a manifold... they are more work to build and make the drivers more difficult for visitors to see (having two or four 18" drivers facing you is pretty dramatic!).
Hope this helps