Agree with Diz, gotta go with powerflow.
I had one on my *coughs* Vecra SRi V6, and it sounded incredible.
You basically drive in, tell them what look you want, how you want in to sound (raspy or rumbly etc) and they bolt a load of their components togeher to give you the xhaust you want!
Give them a go, cracking warranty on them too!