This is Frank owner of Z Performance Racing and the one who builds the header. I am using a real good friend of minds screen name to reply to some of the questions concerning this header. To make 317HP to the wheels is pretty easy and this is how we made it:
Stock turbo Block, Ported & Polished Head, Ported & polished Intake with 75MM throttle body, intercooler, 2 1/2 inch inlet and 3 inch outlet on the intercooler, Complete 3 inch down pipe and exhaust, That header you are discussing, 7300RPMs, and a KKK-26 turbo set for about 9lbs of boost. The stock block with the correct head on it and the stock intake will make 170HP at the flywheel, Now port and polish both head & intake, add large throttle body and you roughly come up with a safe conservative number of 230HP, for each pound of boost you should get a conservative 7HP, that times 9 is 63HP, plus another conservative 60 (but we know it was more) for the header and you are now at 230 + 63 + 60= 352HP to the flywheel and this does not include the air inlet tempature from the intercooler, increased compression, increased ignition timing, & ram air. 15% drive train loss and I think this is easily done. I won't go into details about each part as they are Z Performance Racing entitlements and only for customers.