Go get a real tune from a certified tuner here in Atlanta, or just go the cheap route and get an anti-fouler from AutoZone.

My WRX was tuned by Scott Siegel when he was at SOG, but now he is working with Doug at TopSpeed. BatlGround is also a great place to go for tuning.

Honestly when working on any portion of the exhaust and removing cats in an 04/05 WRX (probably the pickiest years) you should get a fresh tune to get the most out of the mods. Going catless without a tune can cause issues.