No id did my Cat months after header intake. So i did notice the difference.

It added no sound to the car. The headers and intake are the source of noise and mine not that loud.

I have NEVER thrown a CEL. Mines a 300cell ceramic CAT with the probe in the middle of the guts.

Not sure if mine 2.25 or 2.5 inch, either way my header output is about 2 inch so who cares!