I'm not entirely convinced that many of those race car examples even have a well-designed vent. I'm not an expert but here goes:
Let's look at the McLaren P1, a car that you can probably trust has extremely functional aero features, considering how it's advertised as such, and has among the most downforce and least drag of any supercars.
The holes in the hood are way smaller than in some of those pictures if you compare to the intake area. I have a suspicion that a lot of people who make gargantuan hood ducts unknowingly are reducing the pressure over the hood by doing that and losing downforce. That Evo for example, has an exhaust that's like 3x the area of the intake.
I'm guessing the difference here is that air going over the hood of the car is moving a bit slower since often times that's an area of positive pressure, but some of these guys are slowing the air down too much as it leaves.