Ed,
<... Did this help or did I just make it more confusing ...>
Once again we find ourselves in "violent agreement" ... :o)
In your crude "box" plenum, (or any design with too sharp corners) when the air couldn't negotiate the turn, it separated, became a turbulent eddy and stopped going anywhere or doing anything useful.  Smoothing out your ramps reduced inlet from 24 square inches to 8-9.  That's  2/3!!  A HUGE savings in cooling drag.  That is what I was trying to get across.  Merely delivering air to the radiator face is not enough.  We have to deliver it in a sufficiently organized manner to enable it to work as efficiently as it can.  I know 7 deg is hard to achieve in our limited space, but my objective is to have everyone giving it their very best shot at 7 deg, and accepting what they end up with as the best package they can make, rather than just hanging radiators out in the sky.

I was surprised that it was only 7 deg.  I expected more like 20 .... Jim S.