You gunna run 30psi, jeez, ha ha.... still a pretty big turbo, will be pushing a fair volume of air into the engine.. I don't know to much about boxer engine's as mostly dealt with rotor's, which can turn some pretty big turbo's.. would think you'd keep the runners same size as the ports, as only two cyliders will be going into each pulse of the snail. wrx headers are pretty long, would think the exhaust gases would be to slow and lose alot of there heat by the time they got to the the exhaust housing... I'd stay with smaller headers with such a large frame turbo, that's just my opinion, may not be anywhere near the mark...