Have you considered using a motorcycle exhaust silencer with an inbuilt cat? As it means that you wont need to have a separate cat hidden somewhere else on the car.
However depending on the exhaust length this may affect the designed cat light off temp so it not be enough to keep emissions down when the engine/exhaust is cold. I'm unsure as to how stringent the emissions tests are for cars so you may be able to get away with it.
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