How restrictive is the exhaust fitted on a STI PPP compared to say, a milltek or TSL system?

 

Depends on the year. For instance, the system for 03 STi is fine up to 370+ typically. A very small benefit may be gained by replacing the resonator with a straight section, but altogether, it's not a bad system. Later systems (06 on) I believe, retained the standard cat as opposed to the sports cat downpipe of the earlier models.

JohnD