What kind of HP increase can you expect from the STi muffler? I have a WRX.
Free flowing and larger exhaust does more on a turbo car than a N/A car. I have no idea the power gains for your WRX from just the pricey STi exhaust (back box only). This is however the N/A powertrain forum so I suggest you post in the Factory Turbo
forum to receive better answers as that is where the question belongs.