I have Borla Exhaust on my car with no cat, and it sounds great..but if i were to do it again, I would buy ATR's dual 3" cat back system. Its the sweetest thing I've ever seen, and will make a big difference if you replace the downpipe with a 3" one as well. I'd stay away from flowmasters because they are designed for naturally aspirated cars and create backpressure. On a turbo car, you pretty much want no back pressure, all the pressure you need is created between the headers and the turbo.