Someone correct me if I'm wrong, but on the Duratec, one cat is welded onto the exhaust piping after the flex section, and the other is integrated into the header. So I'm going to assume with your FS flexback exhaust you've already removed one of the catalytic converters? If so, the only way to remove the cat is with a race header because (as I mentioned before) the first cat is part of the header.
Both of you are saying the same thing...I think. There should be a cat integrated into your stock exhaust manifold. You alread removed the other cat with the cat back. Since you need a header to remove the cat thats integrated into it, there are no other ways to do that until you get your racing header. Thats if im reading all of this correctly.