I think it is actually illegal to alter any exhaust system in a way that it does not comply with emission standards. In some regimes an aftermarket exhaust system must be stamped that it complies with the standard and even for which vehicles it is approved. In any case periodic MOT testing should establish whether or not the exhaust system is compliant with the standard. Things become more complex with vehicles fitted with particle filters as there is not an MOT test (as far as I know) for this.
My understanding is that moves are being made to use the on board diagnostics for vehicle testing. I had heard that OBD was now being used for testing of vehicles first registered after a certain date in Germany (Heinz might be able to throw some light on that).
It is hard to imagine that anyone would really want to perform any real tuning on the CX series cars for normal road use but if any serious use of on board diagnostics is made for MOT testing on new vehicles it would tend to discourage any such tuning modifications.

Peter

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