Invidia 02-07 Subaru Impreza WRX/STI High Flow Catted Downpipe w/ - HS05SW1DOC

but the point is

it's not as simple as just strap on a drain pipe and gains for days. A "quality" exhaust system maximizes performance gains by design.

Far more engineering goes in to the design of an exhaust system than many realise. From a performance perspective, there is a fine balance between exhaust back pressure and exhaust gas velocity. Too large a diameter pipe, and low down power and torque is sacrificed (particularly on naturally aspirated cars), too small a diameter or added restrictions and bottlenecks in the system, and the engine is unable to push gasses out of the combustion chamber efficiently, reducing power and torque. There are many other technical aspects such as 'scavenging effect'" which are beyond the scope of this document