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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?
Thrustvector-
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
The internal surface shouldn't really matter at all, since what you're aiming for is just a big volume of air, rather than a smooth surface for the air to flow over. The surface would only matter if the airbox was acting as a restrictor, so that you're trying to squeeze as much air through a confined space as possible. If you airbox is acting like a restrictor, you be doing it wrong. In any event, a little bit of turbulence is not necessarily a bad thing. Cylinder head ports flow better with a slightly rough surface than a polished one.