I know this is an older thread, but seems like some disinformation.
1) CEL thrown due to a change in CAT will be because the ECU has found the amount of pollutants is out of param and cannot correct the A/F ratio enough to bring it back in line. The easiest solution and recommended is to tune the vehicle. Simply deleting the sensor will likely leave the ECU in the last known state of the A/F ratio.
2) Test pipes will throw a CEL on a stock ECU. Your choice is to live with it, periodically reset it or delete the sensor through a program like Uprev.
3) HFC due to their construction will be more expensive and theoretically will pass emissions testing and not throw a CEL. In practice YMMV.
4) Free flowing exhaust will produce more HP on the top and mid range. Some engines may see a decrease in low end torque. IMO I would rather have the free flowing exhaust and take the increase in HP.
5) Changes in the exhaust system will result in sound changes. Some good, some bad depending upon your taste. Personally I can live with a little more sound outside the vehicle, but would not want increase in drone. Would also want to carry on a normal conversation inside the vehicle. My goal is increased HP with moderate increase in sound.
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