There are, actually, high performance catalytic convertors available. The ceramic element converters, as used on MB products, do not cause an appreciable restriction. There is a company which clung to the pellet bed style converter for many years (GM) and actual performance gains could be made by changing these to a creamic element style. The enlightened, MB (or DC as they now like to be called) included, have used the ceramic element style for many years, and I'd be surprised if a "hi perf." cat would cause an appreciable difference on a car that is more than likely optomized from the factory. There is a possibility that some gains could be achieved by changing mufflers.