I'm running a Stainless System on my GT, bought from PS Autoteile in Germany, see attached pic. It originally came with twim oval outlets but i prefer the original look.
The backbox is also restricted origainally for TUV, but to get round this you simply removed the sleeved oval pipes and drill out the restrictors.
I have to say that i've had quite a few problems with my system, 2.5" bore and a lowered GT means that it doesn't really do speed bumps very well.......in fact it's been known to scrap even when driving over a human hair
It's also very loud, which 12months ago was unbearable for my little boy, but now he's got the speed bug he doesn't seem afraid of me pulling 7k......on a private airfield of course.