If you look at the flow of water through a radiator, every part it flows through gets ample airflow, even with a deadspot.

I'm not convinced a dead spot itself limits cooling much at all, to be frank.

I'd bet shrouds work () more because:

1) of an actual increase in net airflow.
2) slightly evens out air distribution (horizontally--AWAY from center).


Here's some (theoretical) math:
Let's say a hub (and therefore deadspot) is 40mm in diameter and the actual airflow is 120mm across. If you have an even donut-pattern of airflow of 100CFM, I'd bet the majority of airflow is actually going over the center channels, probably ~70-75% of airflow will go over the middle 50% of the radiator. The outer quarters will thus only get a small portion of airflow (and therefore cooling). This is because top and bottom of the donut will go on the center channels, while the outer edges of the radiator will only get a part of the airflow each. What makes this even more troublesome is that airflow is not an even donut-pattern, it weakens at the edges (with most fans), thus giving even less cooling to the edges.

I can't find my calculator, and I sure don't want to the integration by hand, so this is all theoretical, but just imagining it in your head should help. (maybe I'll draw one and have Photoshop do the math for me )