My 2006 GT convertable has the following mods: 

• Ford shorty headers
• Magnaflow Cat back exhaust with MagnaPack  mufflers
• C&L CIA
• Superchips tuner

I am looking for recommendations for  the next set of mods to make.  The car is stored for the winter due to the salt here in Illinois. I want to gain HP without big reductions in gas mileage.  I have thought about ubderdrive pulleys and a larger throttle body.