Exhaust Headers

What sort of gains can I expect to see if I swap out the stock header for an aftermarket unit. I want to keep my stock muffler. Thanks

1-4 lb-ft of torque, depending on how well the OE header was designed.

Yours gain are higher if your entire exhaust system flows well.

A 2.5RS would probably run a similar exhaust to a WRX which requires much more airflow. I wouldnn't expect much. More for sound than anything else.