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I would argue that he "performance" cars you are thinking of have the airdam more for cosmetic reasons, to enhance the image, than practical. While the front of the cars today are cleaner, aerodynamically speaking, than our beloved musclecars, they are still subject to consumer whims and expectations.

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You are mistaken in that, the air flow is so carefully controlled that some of them will relentlessly overheat if the front spoiler is missing or broken. Even if the cooling fan is on continuously, there is not enough airflow going through the radiator. Some of the more flamboyant packages are aimed at the boy racer crowd, to be sure.

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Then how do they cool at lower road speeds without the airflow that is available at highway speeds?

Got an example of this in production?

I'll grant that the emphasis on clean aerodynamics has reduced grille openings to the minimum possible, but we were talking about aerodynamic improvements not cooling concerns. I still maintain that while an airdam might very well be important for cooling today's cars (not on my DD, it has a pan blocking the chin area) it is still mostly cosmetic from an under car airflow aspect. My DD has an airdam, why? I dunno for looks I guess it is nowhere near close enough to the pavement to be effective in blocking under car airflow in any useful amount and it cannot direct air thru the radiator as it is blocked off above the air dam.

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Highway speed are where they are the worst, all the air is deflected away from the opening, if you look the "grill" area is really tiny in comparison to the gaping maws that older cars have and placed much lower, really out of the airstream.
You would like an example....well, I have one. It involves aerodynamics, performance(for it's time) and a comedy of errors.
The shop I was working at the time was owned by a Father/Son and the Father had a 3rd gen F-body Trans Am that developed an overheating problem-on the freeway. This car was his pride and joy and used to make high speed runs to Vegas, he had a second home there. He is a mechanic and a damn good one as is his Son, so this would not do. It was a matter of Pride as well as need that it be fixed.
After replacing the radiator, fan switch, the cooling fan (including wiring the fan to run all the time)the problem persisted. Next the heads and finally the engine-yes the engine was rebuilt- basically to stock specs..... still the same.
After all that, it was noticed that the small lip spoiler was broken. It was a modest thing only a couple of inches long, running under the body just behind the lower valance opening, but replacing it fixed the overheat.
Again, as I pointed out in my first post, there are cosmetic aspects to these things, but engineers are driven by the need to control airflow and the Mfgrs spend millions to maximize MPG and any package that gets put on a production car isn't going to cause a significant increase in drag. They can offer them through a "performance division", or just let the aftermarket deal with it.
Most of the double wings, body kits and such have nothing to do with the OE and are so poorly built and engineered that they are laughable.
As gaudy as the Wing cars are, they have very good Cd numbers and are very stable @ high speed.
Take vortex generators, those little triangular looking things that are popping up on the roofs of newer cars, with careful placement and sizing they can reduce drag and increase stability a surprising amount. If you just buy stick on ones and put them anywhere they may do nothing or increase drag. So they are only there for looks if someone decided they would look cool, not if the Aero Engineering team specified where they should be.
Ultimately the only way to KNOW if something is truly effective on my/your/anybodies car is to test it. Unless you have access to a supercomputer or a wind tunnel it's more time consuming ie coast-down tests, taping string all over your car and watching/videoing where and how it moves around, using a Magnehelic gauge to measure pressure points, Etc.