Disagree
I have had a cat-replacement pipe on my Fiat Coupe for about 9 months, and it saves me about 10% in fuel costs, and a similar improvement in performance. It's already paid for itself several times over (I monitored how much petrol I used before and after), and so the 20-30 quid for putting my cat back on on MoT tests is also paid for in petrol savings.
Remember - when cats came out, most manufacturers had to make changes to their engines to even return the same BHP and fuel economy - most went for injection. Look at the old XR2i - it was a 1.6 prior to the new laws, and then they released a 1.8 with a cat, and the performance was not improved considerably.
