On other cars, cold-air intake is proven on the dyno. Every Mustang magazine has tested variations (5.0s come with cold air from the factory). Some designs make a little more horespower than others, for example the filter design. But all make more power than sucking hot air from the engine compartment. We're only talking about 1%-3%, but it's practically free. Cold air is denser than warm air, so it's a bit of a supercharging effect so to speak.