With respect to the O2 sensors, I mean turner off in the PCM program. The sensor remains in place for appearances.
Any of the combinations I described will not be all that loud because the stock cat back remains in place. (The stock cat-back is actually hard to be performance-wise, as it is a full 2.5 inch stainless steel system that flows quite well). At idle, if you are outside the car, the sound will have a deeper tone - almost like you've added a subwoofer but decibel level is not much higher. It will be a little louder in the cabin at full throttle but not anywhere near the sound level that aftermarket cat-back systems introduce.