The first thing I would do is a good cleaning and keeping up with regular maintenance. If the cat, the fuel filter, or a sensor is not up to specs, that needs to be addressed before any aftermarket parts are even looked at.
Then I would do gears. :D Gears, gears, gears, gears, gears. Best bang for the buck you could EVER do. The right gears can make a 2.5L feel peppy and help a tired 4.0L smoke a v8 car.