Louder is faster, but on the edge you are out of control.
Slight edit of the classic line from Days of Thunder, but being serious, why do you want it louder?
If you want more noise, an induction kit will give you increased noise on acceleration, but with no drone at speed.
If you want a more sporty rasp, a replacement back box will be sufficient. I think its Irmisher (?) that do that for Vauxhall as standard, so you can probably find them on eBay for a decent price.
Usually people have louder exhausts fitted to improve performance (they are louder due to less noise suppression, bigger bore, whatever), and they don't go louder for any other reason...ish.