I gutted the airbox on a 1998 SE. It made the car noisier, particularly at start up, but gave me no noticable increase in power and no measurable improvement in fuel economy.
In my opinion the factory airbox is not a constraint on either horsepower or fuel economy. In the first case it would only be remotely possible at wide open throttle near max RPM for the airbox to come into play; and in that condition several other factors would be more important.
In the second case, fuel economy is controlled via a feedback system including the O2 sensor(*) and computer programming; so if the system was restricted the ECM would adjust the air fuel ratio to deliver the right fuel economy.
Back in the bad old days (when carburetors ruled the earth) restrictive air intake, usually caused by poor maintenance and clogged filters, power and economy could be improved by cheap and simple means. Today with modern engines, computer controls and CAFE standards, all you'll get by gutting the airbox is more noise.
If that'* your cup of tea, break out the dremel!