It is very good to take a scientific approach. Before going to a lot of trouble however lets use the facts that we do know an analyse them. Frontal area of vehicle feeding radiator about 0.8 x 0.5 = 0.4 m sq area of aperture forward facing scoop 0.15 x 0.05 = 0.0075 m sq. There is a huge difference in intake area so even if the pressures were the same at the front of the radiator and at the scoop the volume of air coming in through the radiator is going to make the amount of air coming in through the scoop appear insignificant. Althought the radiator, i/c and a/c will all act to block the aperture the fan(s) will all be working to compensate.

In the grand scheme of things the amount of air pulled in by a scoop will be like a drop in the ocean. A scoop will have a localised effect as it will bring cold air into the engine bay so what ever is directly in line of the scoop will get some local benefit (top mounted i/c or cold air feed into the combustion system). These local benefits are maximised by segregating the air from the scoop and that already under the bonnet.

As for letting air out! Yes hot air rises however the amount of air pushed by the fans out through the bottom of the engine bay will once again be much greater than any air escaping through a scoop. Raising the bonnet will provide a much greater area for the air to escape that any scoop.

Good luck with it.