I'm a firefighter by trade, so I have pretty extensive training on your example for positive pressure ventilation as it's how we clear smoke from houses. It's not a great comparison to begin with as there are a lot of differences between the examples that make the comparison fairly irrelevant. The air has to flow through and out of the engine compartment, for example, so the shroud isn't even close to the only thing that effects flow if you start considering the whole system. And it's not the biggest. The airflow out of the engine compartment would be a much larger factor for creating any kind of positive pressure and reducing flow through the radiator than even a fairly restrictive shroud. What you're talking about has more to do with the aerodynamics of the car as a whole than a fan shroud.
A few other issues- the fans shouldn't even be running if the car is moving at any kind of speed. If the radiator and the rest of the cooling system are operating properly the fans shouldn't even be on if the car is moving more than 25-30 mph, so all of the airflow at that point is from the speed of the vehicle which should be more than enough volume to be able to overcome a little extra resistance from the shroud. Meanwhile, when the car is moving slowly or not at all the shroud allows the fans to pull air across the entire core. The factory used shrouds for the stock mechanical fans, going without a shroud just reduces the efficiency of the fan regardless of whether it's mechanical or electric.
That's one of the reasons I used a factory electric fan set up. It has a shroud that was designed to work with the fans that it's equipped with over the surface area of that size radiator. Some of the aftermarket shrouds are a just a flat plate with holes for the fans- that's just a bad design. Same deal with fans that come without a shroud, you have no idea if you'll actually get the rated CFM because most likely the rated CFM was achieved using some kind of shroud for testing. If you don't replicate the testing parameters you won't get that kind of flow.
Yes, there are shrouds out there that have flappers and things that would be open when driving and closed when the fan is running. I personally think they're kind of gimmicky, the air flow through the shroud at speed has to be pretty darn restrictive before something like that is necessary or useful. Basically it's a band aid for a poorly designed shroud or undersized fan area.
Yes, you absolutely have to balance the entire system. All the components have to work together for the system to work the best. Sure, you can overcome some amount of undersized fan by running a larger radiator, or overcome a smaller radiator by using a much higher rated fan. Same goes for the water pump, pulley ratios, etc. But for best overall performance and efficiency everything has to work together. Run more fan than you have to and you're losing horsepower driving the fans. Run more radiator than you need and you'll lose space which may require running smaller fans, which could cause in traffic overheat issues when you're not moving enough air. Run too much water pump and again you're losing power driving the pump, plus you may lose cooling efficiency if the water isn't staying in the engine long enough to absorb heat.
If your system works that's great, it sounds like you have a fairly complicated set up with the twin turbos and intercoolers so going beyond the standard kind of set up and thinking may be totally necessary. A 200 amp alternator is something a lot of us don't have though, and something the stock wiring harness probably won't tolerate for very long. I was just pointing out that your CFM requirements, and therefore your amperage load to run the fans, would probably be lower with a well designed shroud. Of course, a poorly designed shroud may not help much at all.
Yes the Black Magic fans have a great CFM rating, but they come without shrouds so there's no guarantee they'll flow that much on your car. Plus, you can get fans that flow more air for half the price and were tested to go over a 100,000 miles.
