heres what you should do:
1. CAI intake, lots of options, see what fits you best
2. exhaust, get a more freely flowing exhaust, maybe dump it or have an earlier side exit in front of the tire instead of behind it
3. Pulleys, Underdog Performance 4 piece pulley set will gain you 8 hp alone. combined with the 2 previous mods, you'll see a substantial increase in hp, and depending on your driving habits, a gain in mpg of between 2-5
4. Electric fan, it reduces the load on the engine and frees up a little bit of power, it can be done cheap with a taurus electric fan from a JY for $20
5. Tune, buy a programmer from Underdog Performance and it comes with 3 tunes, you can get those tunes to get the most performance out or efficiency out of your engine.
with all those mods you should feel a big increase in power and throttle response, you'll gain mpg, most notably on the highway, you'll see at least 5 mpg increase on the highway(i saw 3 mpg from just my CAI). Your driving habits have the biggest impact on MPG though, and if you have a lead foot, all the mods in the world wont increase the MPG.
heres underdog performance