"Leatherman, your idea of shroud helping airflow when standing still makes sense..... however, it strikes me the bloody shroud actually blocks airflow when car is moving. "

That is very close... The shroud is not at fault though... the fan , when turning at certain rpms,but with the car speed above certain speeds , does act as a restriction to airflow.
However, the amount of air which does cross the radiator fins should still be fine for cooling the engine even though some may be blocked outside the car and go around the radiator.
This situation is why some cars went to ONLY electric fans... and some added the viscous clutch to allow the ram air going through the radiator to push the fan past the rpm which it would have been turning due to the belt run by the engine.
If you will check out some professionals who know about air pressure you will find that the area under the front of the car, when it is moving , is a high pressure area... which means without the shroud you have air pressure in the engine area which will also be working against air going through the radiator fins...
I hope you will reconsider your decision to run your car without the shroud.

If you look around...you will find that MB is not the only company using shrouds... so it may not be " overengineering"... it may be a good design based on sound air flow concepts.
It is even more critical to have the shroud if you have an AC condensor because that adds even more restrictions to air getting to the radiator.