I think Yellow injectors are only phase 2? As it's a V2 WRX, they wouldn't fit. Could be entirely wrong about yellows only being Phase 2. Wouldn't be the first time.
Honestly, your biggest and best bang for buck modification on an early generation WRX etc, is 100% the ECU and tune. You're not really going to get a noticeable bump in power by the other small mods, without introducing a lot of risk to the motor due to trying sketchy stuff to get more power.