I'm not sure if this is true. I understand your thought that the vertical flow/fin OEM design would lead to a larger surface area that is not being cooled, however: (let's assume a radiator is 2' tall x 4' wide)
Vertical flow/fin OEM Design
-The water going through fins that are cooled by the fan are moving slower so they are being cooled longer (4' wide traveling 2' vertically)
A cross-flow (horizontal fins) radiator
-More of the fins in contact with the cooling ability of the fan, BUT the flow is moving much faster so the water has less time to cool down when in contact with the fan because the flow through the horizontal fins is higher (2' vertically travelling 4' horizontally)
Keep in mind that fans are pretty much only good for stagnant air/stop and go traffic. With that in mind, the surface area of the fan (which is increased by a shroud -but also adds a restriction at speed -on another side note, fans themselves are restrictions at speed) and the CFMs of the fan are the two major aspects we are looking at. If we use the same diameter fan in both the vertical and horizontal flow radiators above, we have the same amount of cooling surface area interaction that the fan has on the radiator. So the question is:
-Does a slower moving flow of the vertical design cool better despite a larger surface not being cooled by the fins, or:
-Does the horizontal design cool better when more fins are in contact with the fan despite the faster moving flow?
The goal is to use the smallest surface area fan with the lowest amp draw that will suffice for the worst possible conditions. I appreciate all of the hard work and my input is intended to help guide proper testing so I know what to put on my personal car 
On a side note, I don't think the vented hood has a significant benefit on cooling in stagnant air. I think a stock non vented hood (as long as there is no flat-bottom tray that blocks off the factory radiator exit to the ground) would not be any worse to deal with the volume of air exiting the fan. Where a vented and properly ducted hood makes a big difference is when the car is at speed and there is far greater flow than the fan can provide (which is why a fan is a restriction at speed).
