If you are wanting HP gains with an air intake system under the bumper, that is NOT going to happen?
Any airflow in a tube or hose will be restricted to a degree, and a lot when you add bends as you then add more restriction.
When we were running a few turbo cars in NHRA class racing where there is very close competition and every single HP needs to be available, we did lots of dyno testing in stock and modified turbo cars. We found 12-40 HP difference from no air inlet ducting to various ducting configurations.
The best configuration when rules allowed, the turbo would be oriented with the inlet facing the front of the car and at high speed the ram air direct into the turbo would add HP.
In the case of a street GN, the air temp under the hood becomes the same as ambient air temp as soon as the car gets moving, so this negates the thought of incoming air from under the bumper being cooler.