I too have a friend.
On his Elise he has had the exhaust swapped from "normal one with Cat" to "noisy one without Cat".
He assures me that this conversion is totally pukka and is carried out by the supplier of the exhaust system, who also gives him the original for when the MOT is needed.
Shurely this is more worthy of PC Plods attention if not for the noise then certainly for the fact that his emissions will no doubt be over the intended limit.