My old VS 165kw was dyno'd with a hollow cat, only made 116rwkw, at that time the average (stock) VS was around 108rwkw. It did make a distinct hollow ringing sound with the stock single system. Thinking of it, a hollow cat is probably the worst thing 'flow dynamically' wise to have. I would imagine any serious performance increase would require a complete package. (Again the VS - pacemaker pipes, twin cat, twin 2.5 straight through mufflers and the chrome tip only gave 24 rwkw extra - with std chip) As for them knowing what's inside, I do believe that having a working cat is not even part of the QLD roadworthy check, as long as something that formerly resembled it, is there. As for the polution, havent I heard which 'old' cars can run on unleaded - without cats.....same polution, if not more. JM2c Jim