Hi,
many people have done it, they seem to enjoy it
I haven't done it myself though.
It is not legal to fit the exhaust of another model to your Cooper S, even if the engine is quite similar. But as there is a MINI part number on it, most police officers and TÜV engineers will either not even notice or just let it pass either way. In that case it's your choice if you want to do that.
regards