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Talking “special” air filters can bring out some strong opinions, but I will offer my suggestions. If you are talking about the oiled element cone/high flow filters, they are great for letting little particles into your engine. I prefer a real paper element filter, of which there are high flow versions if you are so inclined.
Since you have a gasoline engine, your biggest flow restriction in your intake system is the butterfly valve driven by your throttle, so to have a low restriction air intake system you will need to keep your throttle fully open at all times. In all seriousness, you would only see the gains from reduction in intake flow restrictions at or close to wot, as the actual job of the throttle is to throttle back (restrict) the amount of air going into the engine. Remember your car is constantly trying to maintain a certain air to fuel ratio.
With some vehicles, the MAF or other airflow sensors are relocated out of the factory parts into the aftermarket parts, which typically had larger diameters. This change caused the meaning of the readings the sensors were returning, and correspondingly caused the cars to run rich or lean. If it made your car run lean, you could actually see a slight bump in MPG, with the downsides associated with running a car engine lean.
Sucking air out of the engine bay versus the fender is a version of a “warm air intake” which could help mileage, by reducing the amount of oxygen in a given volume of air due to the increased temperature (remember fixed air to fuel ratio) but will hurt power.
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