This question has been asked time and time again. Even a FC hi-flow cat will die out eventually. Some people have had the combo and not had any problems, however; others have had their FC hi-flow die out just as quick as the stock cat.
Might I suggest you buy some race headers, and save yourself some money and gain some more horsepower. Just put your stock header and cat back on when it comes time for the e-check.
I have a pair if your interested...sorry fellas I couldn't resist.