I would say that anything like that should be inspected. Anything that modifies the safety integrity of the car needs to be right.
However, unless it is something that screams unsafe, I don't think it would be noticed or commented on.
Unless and/or until something goes wrong!
The new regulations for the MOT coming in for next year will be a lot stricter and things like blocking out brakes to bigger discs will be a failure, as was explained to me by an MOT tester. Also they will physically check to see a cat is present rather than just rely on emmisions.