It kind of ramps down toward the end. The spring on those wastegate is pretty soft, so that can be fixed with a helper spring. That should keep it linear across on the top end. I don't really know why the power aren't higher. I would think it would be much higher at 15 psi. We checked for boost leak before and it checked out good, and the turbo has a fresh rebuild. No damage to turbo components prior to the rebuild. Any clues..................?