I expect most gains from "cold air intakes" are actually from a design that flows better. A better term may be "high flow intake" as the factory ones are often compromises in cost, noise, packaging... I think the main reason they grab air from the fender is it is cooler and quieter but the hole is often smaller than the throttle body. The fender probably does not have as much air pressure building in it going down the highway as under the hood so there is probably a small gain from that. The factory likes accordions for convenience both on install and design and they don't tend to flow as well as a smooth wall tube.