The limit you're hitting isn't caused by the waste gate.... not precisely. You should be able to flow upward of 13 - 15psi before hitting fuel cut. If it's cutting out as low as 5 or 6psi something is wrong. Check for leaks in the intake plumbing, fouled spark plugs and check for diagnostic error codes (most likely either AFM related, abnormal turbo pressure, lean conditions or knock).
What boost gauge are you using and where is it connected? You should be hitting at least 5psi without a boost controller, even on a weak waste gate actuator.
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