I had a Veilside exhaust on my Celica, and that thing was loud enough to wake the dead! It would set of car alarms when pottering through multi-story car parks at normal speeds. It suited the car though in some ways, as the car was much 'louder' in appearance than the Leon.
However, it gets very tiring, very quickly. You can't have a conversation in the car without shouting, long journeys are a nightmare, motorway travelling gives you big headaches (literally) and after a while you just wish you could shut it up.... but you can't.
Ended up changing it for a TRD system which was subtle, yet sporty