i don't have a cat on my honda i have a b16a jdm with headers and apexn1 cat back and i was told that you will lose gas mileage (which i did) and can make the car idle a little different. i don't know how true it is but i talked to alot of guys and the answer was the same. We are just getting a visual emission test so i am forcded to get a high flow cat so i will post the results soon.....