I doubt you will see any benefit..
First you would need to understand how a NACA duct works. The shape creates opposing vortices, in otherwords twirling air on each side of the NACA duct and this opposing twirling airflow on either side of the inlet pulls air into the inlet. The is not a ram air setup, a NACA inlet is completely different. Ram air inlet is typically a scoop which grabs air. Ram scoops need to be positioned higher than the low air speed boundary or right in front of the fascia. Whereas a NACA duct draws air down into the duct like a vacuum. Engine vacuum will promote the vortices and will help draw the air in. NACA ducts are not known for creating high ram pressure. If you had a ram air scoop I would then suggest a wind tunnel test to measure increased pressure. The key benefit of your NACA intake setup is pulling in cold air (not from the engine bay) without significantly disturbing the aerodynamics of the car (like having a raised ram scoop).
See this post.. the race GTS-R has a ram air setup, collecting air from the grille opening not with a NACA duct.
http://forums.viperclub.org/rt-10-gts-discussions/629926-some-gts-r-pics.html
Take it from a Canadian eh'..
Regards,
Mike
PS - Dave.. I thought you bought a book on aerodynamics or race car body design year or so ago?