You actually get worse mileage with the FWI because you're pulling more fuel due to the colder air. But you make more power. It's kinda a win\loose situation. If you want a setup for the track and street go for a Short Ram intake. You will pull the air from the engine during normal driving then when at the strip pull the drivers headlight out to get cold air into the bay.
Otherwise if your going for MPG's you don't want a FWI.