This was taken from the KB web site, and should give you a pice of information to make a more informed decision on the headers.
"Long tubes make very little, if any, power or actually lose power according to our customer reports. They eliminate the pre-catalytic converters. That isn't 50 state legal. And don't confuse any power gains from removing the pre-cats with header gains. Pre-cat and cat removal will net around 10HP. In our opinion, headers for the 4.6 GT aren't worth the small peak power gains and expense. Always keep in mind that any power gain from exhaust (headers, cat back, and cats) is incrementally proportional to engine air flow/HP and reduced back pressure. That's why some products make "0" Hp on stock or +100HP engines but more on a +200HP engine. Every component has it's limits. Example: If the headers make 20HP at 700HP but nothing a 400, 500, and 10HP at 600HP why would you buy headers for an engine that will never make 600 or 700HP?"
Just my .02 cents.