As far as I know you cannot have boost lower than what your wastegate spring is set for. Unless you can figure out a way to make the wastegate open sooner.
I would probably just purchase or borrow the proper spring for 6 psi to be sure you'll have the proper boost level.
2004 SVT: bewst'd!
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2013 Escape SEL 2.0 Ecoboost 4WD
2004 F350 6.8L V10
1986 XR4Ti 2.3L Turbo, 15 psi