Originally Posted by Boss_302

Scruffy,
My thinking was the same as yours, with the ducting to the radiator; I bought a pair of these thinking it was part of the Nismo conversion. Since it was in the dealer Nismo parts list, but in reality it’s the same as the base, I also did some cross checking and can't find anything different, Usually I been finding the part description will say (w/Nismo ) if it is part of the factory conversion.
Does this become critical for normal street car? No! As long as the perimeter of the radiator is seal so airflow will not by-pass it, air will find its way to the radiator at speed do to air pressure differential , also there is a fan on the backside of the radiator to pull air through. If this was an open wheel car aka formula type, it’s a different story. A ducted opening with zero by-pass to the radiator core was critical in decreasing temperature at speed.
What I would do is rivet some plastic or aluminum sheeting to inside inlet of the bumper cover grill area to make up the difference, make it so it overlaps the inner splash shields and seal it with some glued on foam strips.