Yeh as Chee said mate, dont touch the boost without a remap! If your on P plates and drive an 06 STi you've obviously got come cash. Go and spend 4k on a tbe exhaust and retune.
I run 15psi atm (change of turbo will fix that) but there are other 06's running 18-20psi (but have supporting mods too). There is no need to run 18-20 psi with a good tune!

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365hp and 515nm

New Beast to arrive April 2015 - 365hp, 450nm, 0-100 in 4.4sec. AMG Goodness!