Going to bigger fuel injectors usually means you have done extensive mods and have maxxed out your current setup. If you bolt a bigger set of injectors to a stock or mildly modded engine, you will gain exactly 0 hp, and one big tuning headache.
Here is how you can tell if you need bigger fuel injectors: Take the car to a dyno and see if your A/F drops off significantly at high RPMs. Especially true if you have some ability to tune A/F, you won't be able to richen up. Exactly what happened to me back in December - lean above 6000 RPM no matter how much we tuned.
Bigger FI should be VERY low on the list. Chips work very well on ATX cars, since they can adjust and firm your shift points. You can go with Superchip or Diablo. Sucess depends on your particular PCM code. Chip should be pretty low on the priority, unless you want to keep sending it back for a flash every time you mod.
Otherwise, what Rynn said.
