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Ram effect is going to be negligible thanks to Senor Bernouilli and his principle, at least without extensive wind-tunnel testing. The main benefit is lower intake temperatures. Cold air is denser than hot air, so if you can get cold air into the engine you can get more air molecules into each cc of capacity. Add some extra petrol molecules and you get a bigger bang.
The NACA duct in the bonnet of my racer wasn't even a proper NACA duct (the wall profile was wrong and the edges weren't sharp enough). It was only there because it looked better than just having a hole in the bonnet. At least, I think it does:
Brommers
Thrustvector
Apologies​ for the dense question, but what's the advantage of the airbox over simple intakes in the engine bay - do you get any static pressure recovery from a ram intake?