although i can't live up to that explination, my gt6 competition preperation manual has a graph from kastner of stock manifold vs headers, and the stock put out 4 more horses up to 4500 rpm and then the headers put maybe 8 horsepower over the stock at like 6000 rpm. overall, i drive my focus between 1000 and 2500, so personally, i wouldn't get a header since i wouldn't see the horsepower advantage (more like a loss).
sorry for the comparison between tr6's, gt6's and focus's, but i've never driven my gt6 or a tr6.