Are you using an aftermarket boot gauge or the OEM one? The OEM one is totally inacurrate.
If it's actually boosting to 15psi then something may be wrong. Maybe the leaky OEM BOV just masked it. On the other hand I ran my 3S at 15psi for four years no problems (well at least no problems due to 15psi).
I'd just get a boost controller and be done with it.
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