The air under the hood is quite hot, especially during a hot summer day. It doesn't matter if the car is moving the temperature will be higher under the hood anyway, remember that the air is first heated by the radiator and in some cases oil coolers, A/C condensor, and intercoolers before it gets into the engine where it's heated further, probably with the same amount it was heated by the radiator. 60-70 degC isn't uncommon under the hood, which means that you will loose some power.
Basicly, if we can take air that is colder and denser the better it will be. Rarely any new cars take hot air from under the hood.
The length of the itake pipe has also a tuning effect on the intake.
The gain from colder air will depend on the engine power, it's a percentage number so to speak.