I have a mock up z tune bonnet on mine, rarely get hassled about it then again my car is all black so it doesnt stand out as much. pretty much another carbon fibre or fibre glass is illegal to have unless it is ADR approved mentioned above. i'm unsure of the genuine factor of a v spec / z tune bonnet but i can imagine getting your hands on a geniune one will cost you an arm and a leg!
oh and if you were to get defected for it, the only way to clear it would to put on your stock bonnet and clear it with it on, bonnets are very easy to take on and off and swap, only 4 bolts. just need the extra hands there to help put it on.