I have the catalytic MAC Prochamber due to emissions testing. It sounds pretty good, but isn't that much louder then stock. If you are willing to wait, you are more then welcome to stop by in the Spring when I have my GT out, if you wanted to hear what it sounds like. It cost me $600.00 (after shipping, taxes etc. from 2KoolPerformance) before installation (would have done it myself, but it's a pain without a proper lift).
For shop recommendations, I recommend:
Spenc at 2KoolPerformance (in Prescott) (GREAT guy to deal with, very customer oriented, and fair prices)
Mike at No Limits (in Ottawa). Another great guy. Fair prices. Doesn't do installations though.
Also, Steeda.ca MIGHT be good, but they are horrible at responding to emails lately, so you be the judge.
I WOULD NOT recommend:
Sterling Ford in Ottawa. There is nothing good about this place, don't waste your time/money.
and
Stalker Performance in Ottawa.
Hope that helps you out some. Any questions just ask in here or PM me.
Matt