I'm not sure with a turbo car, but everytime I've heard a car without a cat, its louder and raspier/ricier sounding. The cat kind've acts like a resonator as well. Why don't you try a resonated test pipe? You'll get the flow of a straight-through test pipe, without the horrible sound. Again, it may not make as much of a difference on a turbo car as it does on a n/a car, but if it does, it'll be well worth it.