i had just a milltek cat back. Then i got the downpipe/sports cat a few months later. I was really surprised how nice the cat back sounded when i got a mate to drive past me. However you would never know from inside the car unless you put the back seats down because it must be really well sound proofed?? When i got the down pipe the turbo whistle was louder and so was the exhaust note so i recommend getting the full system. Im now considering the decat so the turbo will be even louder but anything will be loads better then standard!