how about taxing the exhaust system off a stock svt? i should perform as well as any other aftermarket cat-back and i would assume it would be pretty quiet. never heard of anyone doing this though... wonder if it would work.
The SVT header and cat will not bolt up to the stock zetec flex unless you slot the flex bolt holes. But then, the issue arises that the cat is a 2.5 in diameter tube, while the stock zetec flange is 2 in roughly, or the 2.25 in aftermarket flex.
If you can modify an SVT Borla to fit a Zetec flex easily you should be able to do the same with an SVT stock catback. Problem is you'll be buying it cut in half so you'll need it welded back together but that isn't too much of an issue if you take it to a muffler shop.