I have Borla headers, 2 1/4" test pipe w/ o2 sensor ports, 2 1/4 piping to a 2.5" STi Axle back muffler.
SOUNDS LIKE CRAP! I am buying new CATS as soon as I have an extra $571. It sounded way better with the cats on with the same setup. my old cats took a dump on me. They sounded like there were a bunch of nuts and bolt rattling around in there. Not much improvement with the test pipe.
I have to admit, I felt a big power difference when the cats were removed. I'de say an easy 5-10 horsepower to the wheels. I was very supprised.