A true cold air intake, in theory, can reduce gas mileage. The colder more dense air causes the computer to richen the mixture to compensate. That being said, I instaled a K&N filter inthe stock air box with a Smart Madness silicone intake tube, and my mileage went up about 2 mpg. Take that as you will.
There have been a couple of members here that have done a "hot air intake mod" to pull the hot air from the engine bay in and have claimed significant increases in mileage because the computer leans out the mixture due to the warm air.
Take this for what you will, I personally prefer my setup.
Good luck with whatever you decide.