My comments in red. Be careful not to describe the elephant by feeling just one part.

Al G.

If I'm understanding you correctly, it appears that you need dynamic pressure ( flow) that turns into a high static pressure (at the Rad face).

To maintain this high static pressure, the dynamic flow must be free of turbulence, which is associated with flow separation from the duct walls. Hence the need for proper divergent angles.

There must be good pressure drop across the Rad,  not too high or you lose heat transfer, not too low as to create excess drag (an over-simplifcation, and likely the reverse is true, high pressure drop will results in hi drag because it takes all the energy out of the air, low pressure drop would generally be less drag, but my mean insufficient heat transfer area). There must be some turbulence within the duct fins to enhance heat transfer, but not too much as to create restrictions.

I still feel a low pressure area behind the rad would be beneficial. (it would, flow through the core is a function of the delta P; unless otherwise limited)