A3 remapped, replacing with small exhaust
 

I bought an A3 and looking through the reciepts i found it was remapped.
Today i have taken delivary of a replacement cat and discovered it was smaller in diameter than the on that is on. I assume that more work has been done than just a simple remap. can i place the smaller diameter exhaust on the car to get it through its MOT, will it damage the car?