[Monkey Wrench]

perhaps it's a trade off, the fan may move more air through the rad with that plate in place, Thats probably why it's there, but if the fan isn't operating until it gets switched on by the thermostat , maybe it would switch on earlier with the airflow being limited by the flat plate. perhaps it has a faster warmup time by blocking flow like that?
my 67 volvo 122 ,(AKA a Volvo Canadian) had a radiator blind with a chain you pull from the inside so you could warm up your car in cold climates. It was designed by swedes who see colder winter temps than in Germany.

Common practice for those who live in cold areas is to use a chunk of cardboard and block part of the rad. If it's cold out you want the rad to get warm so you can have adequate heat and defrost capabilities. some add block heaters to heat the oil or water to get them going when its cold out.

if you only envision the car going down the highway the fan probably isn't even necessary as you have airflow. in that case yes not having the flat plate might offer more cooling. In that state the fan might be operating but basically being driven by wind as well. Then if you go up a steep hill you may go slower but require more power, and need the cooling and so the fan is needed.
When you stop in traffic it would be a concern pretty quickly without the fan, so it's necessary.

perhaps the requirements are a bit different for someone out in the country that doesn't do city driving as compared to a person living in the city, and same for people living in different climates, but the rad design needs to encompass all driving conditions.

more fans or bigger fans mean more work for the alternator and that uses more fuel. when the battery has no demand to be charged, then the alternator doesn't put as much load on the engine via the drive belt.

i suppose one could engineer things so the fan wasn't powered when not needed or things like flaps could open when driving faster to take advantage of the airflow already available by wind, but of course that would add more complication with very little fuel savings.

the temperature switch is a bit of a tradeoff as it will reduce the reliance on the alternator slightly if it doesn't run when the engine isn't warm yet. It also doesn't do the engine any good to be taking longer to warm up to operating temp. lots of cars did not have electric cooling fans so they are pumping air over the rad when whats actually desirable would be for no or little airflow to allow the engine to reach operating temp faster. some have a clutch on the fan or some add fans with flex blades to reduce unnecessary load.

If you made holes or removed that flat plate it might improve flow on the highway, but then on a cold day it may also take longer for your engine to get up to operating temp. someone in Arizona might have different driving situations compared to someone in Alaska. Racers of course would have completely different requirements of the cooling system.

cold winter driving conditions may be less of a concern for many as it's a sports car, they may garage their cars in winter , It might not be the best choice of vehicle if you are living in a very cold climate, low ground clearance so not great in the snow. all the same , some do need their cars to work right at low temps in winter too. cold doesn't necessarily mean snow.