I had a Ford Fiesta in the early 80's and replaced the catalytic with a straight 'tester' pipe. That made that little car run much better and it would still pass emission tests. The only problem was that if would backfire under certain circumstances and it sounded like a rifle shot going off. I don't really know about the performance of the newer cars  and the effect of the catalytic, but you don't see them on race cars. ... View more