it could be from either one. If you elminate your cat, you have more straighter airflow which makes your exhaust free flowing. Also by elminating your cat you gain power up top but lose some down low.
As for the exhaust, the minimum isn't 3". Sometimes that is too large for the amount of boost you are running. 2.5" is probably more ideal for running 6 psi. Higher boost is for 3" or maybe even 10 psi.