cowie,
its the fact that the ecu still thinks you have the OE exhuast on. the sports cat is less restrictive so the levels of co2 etc coming out the sports cat are greater than would from an OE cat. ECU thinks your cat is knackered and turns the engine light on.
you may be lucky, i dont know, just be aware that if it does come on it wont be a total surprise. mine never came on, but once it did it keeps coming on every 1-200 miles if erased.
It is only a light, it doesnt retard the timing/fueling of the car so its no different. But its annoying having the EM light on, and if you do have another prob it could mask it as it wont show if light is on due to exhuast.
Trusty paper and marker pen could come into play haha