haha,,, I love how its a crime to take off the cat but yet, theres a million high flow cat/testpipe on ebay for sale!!! Anyways, regarding the cel,, No, it will not come on there are no 02 sensors after the rear cats so the car will not pick it up.. a few members here have taken off the 2 rear mufflers/cats and ran a straight pipe... Regarding the Gt3 trade,, I think its do-able. really depends if you wanna jump from a practical car to a non-practical car.. For me its not do-able but for alot of car enthusiast that dont need the back seat, its definitely do-able especially for a guy like you,, who seems to like to track his car... My F is like my family car,, I even have a baby seat in the back... lol