It depends if you want to have piece of mind or not. I put a cold air induction system on my 05 STi and it was running fine (over fueling but very safe) - but in doing so didn't give any BHP increase only a very nice noise. I decided to have a Tek3 EcuTek remap done and the car is now fueling correctly and absolutely flies. I have a magnex cat back exhaust, k&n typhoon induction kit and the remap and the car is making 300bhp and 290ft.lbs of torque - and that is with all three cats still in place.
Dave