We're talking about air temperature, not pressure, and I doubt that the air coming from under the car after passing through the radiator and then the headers is cooler than the air coming directly from outside.
Regarding pressure, the area behind the rear window is under depression, and the underside of the car is under pressure, BUT with front and roof spoilers, it's much less obvious, and when it comes to aerodynamics, intuition is often misleading, even for experts.
For the roof spoiler, I think there is a position to respect and that mine was installed by a previous owner too far forward, but I did not want to plug the existing holes. So it is there more for aesthetics than for efficiency