The Check Engine light will be on all the time so you will never know when a minor problem is detected that can turn into a serious and expensive one. You'll be polluting the air the rest of us breathe. If there was any benefit whatsoever to doing that other people would be doing it already. You will not gain any fuel mileage or performance. The engineers have already accounted for any added restriction to exhaust flow. That's why catalytic converters are larger in circumference than the rest of the exhaust pipes.