What is the best exhaust system to get for my supercharged V6 VT Calais?

For a quiet/performing system, I would get an ex-hsv item from a similar vehicle (VT-VZ). In saying this though you would need the cut the tips down a little as the HSVs have a larger rear bar. If you can't get a hold of one of these units or want something that will perform better (and also be louder) I would consider a redback exhaust system either twin 2 1/4" or 2 1/2" with the rear resonator.