With Strömung units, the rear section kit alone will provide approximately 20 ft-lbs torque improvement from 3200 rpm, about 3-400 rpm earlier spool-up than the stock exhaust. The horse power gain is about 10-12hp all the way to 7000 rpm. These are significant numbers, especially considering the motor will not code or otherwise run improperly. It really brightens up the motor.
We use a 2.5 inch diameter pipe that is capable of flowing 300hp. We could use three inch, but it is not wanted a normal road car setup. We found that the earlier spool-up of the turbo was lost with 3 inch pipe without any gain on the top end. This is a serious flaw to using 3 inch pipe. We give away too much power below 4500 rpm for no additional gain on the top end.
Just like when we choose to run the RS 2.5 motor on 2.25 inch pipe, we have selected 2.5 inch as a practical pipe size for the US WRX engine that is known to do the job. We offer smooth mandrel bent mid and front pipe sections, the latter eliminating the third cat for track sessions. Combined with the higher muffler flow, it also aids the top end power. It is a very simple solution. By lowering the pressure after the turbocharger we are able to get a larger pressure delta over the turbine blades giving more power. The front cat pipe improves power from 6000-7000 range, but it is keeping the 2.5 diameter and the change of the rear pipe that provides the low end torque.
The main CAT that you can see off the turbo is not a restriction under 300hp, so we ask everyone a) leave it on, and 2) do not lust after a larger unit as it is not a problem. The same CAT we have is on the 280hp STI Japan cars. Leave it there. If we had 100 octane gas I would be selling them all day long! But our gas flow at 250hp is not critical with the turbo main CAT in place. We do offer a larger capacity main Cat down pipe if needed.
We have removed the front pre-cat and found the motor makes slightly more power, but needs to have a new ECU setup for the engine to run properly. Given the expense, the limitation of our 91/93 octane gas and the fact that the transmission is rated for 250hp, we advise taking a wait and see attitude towards further modification. Just like the Mass Flow sensor failures of 1999 2.5 RS when fitted with filter kits, we are waiting to see how reliable higher power levels are with pipe, turbo and ecu changes. This issue is discussed in greater detail elsewhere in the site (Big Power) (WRX Engine)