Grumpy you are spot on there, Originally made by the specialist stainless exhaust bods for systems that when fitted obstruct the fitment of the lambda in the car. You can also get right angled versions. I feel that the cats might well be adequate to do the job but when fitting a larger bore exhaust system it allows the gasses to flow through faster due to decreased back pressure and the lambda sensor picks up quantity of Co2 and therefor sends fuelling messages to the ECU. This adjusts everything re. the fueling side of things and ups the Co2 through flow even more. However if it is still within the ECU parameters (which are functional parameters) it won't flag an engine warning light but will show high on testing.
