Psi is just a measure of pressure--not how much air is flowing into the engine. How much power you make from one turbo pressurizing the intake at 10 psi is not necessarily going to be the same as another turbo pressurizing the intake at 10 psi. The one which can flow more air at that pressure is the one that's going to make more power. How much power the motor makes also depends on intercooler efficiency, what sort of exhaust you have, etc.

That said with your typical T3/T4 applications available in the Drag, Revhard, etc kits on a properly set-up LS motor, 10 psi will yield around 230 whp given proper tuning. So for 255 whp figure like 13-15 psi assuming the rest of the setup is up to the task.