I recently installed a Ansa 3" cat-back exhaust system on my 2005 Subaru WRX. Just before doing so, my engine light came on due to what I believe to be a problem with the oxygen sensor (i.e. car is idling roughly, running lean and the DTC codes point to air-to-fuel problems. I ordered a new O2 sensor but was sent the wrong one so I've yet to fix this.
In any event, here's my issue... This may just be in my head or the oxygen-sensor problem has just gotten worse, but the engine felt significantly weaker in the higher revs (after 3,000 rpm) immediately after installing the new exhaust system. It basically feels like the turbo has no umph. Fuel-to-air problems aside, could the drop in turbo performance be caused by lack of back pressure? I realize the OEM exhaust has a second muffler (or is this a second cat) after the catalytic converter and the new system has a much larger pipe diameter. If this is in deed the problem, could this be resolved by wielding a MagnaFlow XL muffler (part #13742) just after the cat?