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i've never heared of a naca cowling generating thrust. it did however cut down drag greatly when it was introduced, because it allowed for both better cooling flow and reduced drag. previously it was believed that the best way to cool a radial engine was to have all the cylinders exposed to the airlow... most cowling that did exist before that where more wide and there to protect the pilot in his open cockpit from oil splatters (especially in rotary engines wich where radial engines where the entire engine turned with the prop for better cooling!)
because drag was reduced so much planes saw an increase in top speed, and because improve cooling meant leaner mixtures more HP was on tap from the same ammount of fuel.
However the p51 uses an inline engine, with water cooling and the radiator is indeed mounted in a spacial radiator housing, that was developed by North American, although the idea had been tried unsuccessful by Curtis. the shape of the radiator housing is such that the hot air that exits does actually provides thrust, even so much that it will cancel out the drag induced by the radiator. This was called the "Meredith Effect".
All of this happens at rather high speeds compared to cars, and what's more a plane is always moving, even when flying "slow", these planes still go faster than the top speed limit for cars... a car needs proper cooling when it's crawling along at a snails phase, to a completely sealed cooling duct might call for a fan to assist airflow, canceling the benefits.
that said there is possibly quite some room for improvements in the cooling departement
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