hi, i'm running a 3'' downpipe and de-cat then its 2.5'' from there with only one silencer at the rear. i've had no problems at all and as myki said the gains really are great, you will notice a much faster spool time. on the down side u will get a bit more noise, on tick over its not much different but open it up on the road and you will have a completly different beast! but i guess it depends on what you use your car for, mine is not a daily driver so noise is not an issue, but its worth thinkin about if u are goin to do lots of long journeys, etc.