I put the Power Effects on my car a couple of months ago. It probably varies by car/engine, but the resonance is very significant, especially in the normal driving range of RPM's.
Also, I had thought that with the butterfly closed (e.g. close to stock exhaust flow), it would be close to stock sounding. Nope.
It goes from pretty loud closed, to really loud when fully open. Unfortunately, not sure how you know if it is acceptable or not without doing it.
Now the good news. A real quality system. I really think I can tell the difference in the cars performance. I don't mind the loudness so much because I do not have to drive it daily or commute with it. Usually, I am just "cruisin", and kind of like the sound.
p.s. I kept the old exhaust just in case, and suggest you do the same. Maybe good to have if you ever sell the car.