[cotbu]

Are you spiking to 15psi or is boost holding steady to 15psi?
Before you modify the the turbo permanently, disconnect the wastegate flapper and go for a spin, document boost. If your boost is anywhere under the 15psi and holding steady, your wastegate is too tight adjust it so it just slips over the flapper arm. Go for another spin document boost.

It could also be your vacuum/boost pressure routing, a pinched wastegate pressure source could make you overboost.

A boost controller will only raise boost over the wastegate setting. It cannot lower it beyond wastegate setting.