Richard, as far as I know the opening underneath between front and engine is unaltered open since the 70is. The amount of air in this region to suck upwards behind the radiator is much bigger than the louvres, different engine size etc., which in my view seems to be more a fine tuning in relation.
But it could perhaps happen that the above mentioned effect is even bigger since e.g. the 1.6 Sigma engine is even smaller and more backwards mounted than a Trad Plus 8 which should result in a correspondingly bigger air gap.
There was a short film somewhere here, I recall also an American one, where the airflow on the outside of the car was tested by using stripes of fabric and you could clearly see where is low pressure and where is more pressure. I can‘t find it. The cowl was in my remembering more a low pressure area and the region behind the front wheel over the wing where the louvres are was definitely a very low pressure area (regarding air intake solutions).
Interesting also what Button writes, perhaps one must „help“ the radiator to get air flow through it with such an additional radiator. But first I would see what Guten gets as a result of his additional floor plate because for me it would be the more elegant solution.
Let us remind this thread is partly about air intake and partly about engine coolant. My contribution here is about engine coolant with priority...in my case the air intake would have to be altered if the engine coolant would better by using a floor plate. But that would be the next story.
BTW its all about the fun, it feels in a positive way as if we would improve our model steam engines from 1970