Interesting question; I hope some one can come up with an authoritative answer.
Since the raison d'etre of NACA ducts is to minimize disturbances in the air flow, my guess is that they won't make much difference. But "minimize" is a rather vague term, so maybe they will make a difference at track speeds. Guessing again, if they do make a difference, it seems the added resistance at the sloped front surface would create some down force. But I'm no aerodynamics engineer. I'm just here to see if my guesses are right.
