Main issue with tubular headers is a loss of spool especially on the supplied up-pipe hence the turbo match Harvey Smith versions available from Alyn! Most people say stick with ported up to around 400/450 Bhp it's only above that you will see gains. I was running 440 on ported OE with matched up-pipe and gained 2 Bhp and lost about 100rpm spool when I switched, however the boost was 0.1 bar lower for the same result (And the clutch was slipping!).
http://www.southeastscoobies.co.uk/v...&postcount=184
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2005 Spec B 6MT - KiDO tuned (~270Bhp)
Custom exhaust - Cosworth filter - Samco Intake - Tints - Bilstein B8/B6 & STI springs - SuperPro ALK - Whiteline positive shift kit, rack mounts, rear adjustable camber bushes, front ARB mounts & strut brace, 20mm rear ARB with AVO mounts, - Hardrace Gearbox and pitch mounts - Prodrive PFF7's - Front Brembos - Kenwood CarPlay - 1/2 carbon grille - STI front splitter