I had Andrew from AM Performance do the cat back on my 370GT Sedan.
In hindsight, I think it's probably better to buy one from a company that has done a load of R&D to perfect their product but Andrew does top notch work and I'm happy to support local business when they're great people.
I'm also happy with what he produced
However with the aussie dollar being so piss poor at the moment, I'm not sure buying from overseas is a great idea right now....
One advantage of getting a custom exhaust made is you can always go back to the shop to get it tweaked if you're not happy. Just try to be as specific as possible when you tell them what you want otherwise you'll end up paying even more.