just so you realize, when your motor is naturally aspirated, the power band doesnt drop off at the top do to lack of air. even at normal atmospheric pressure the motor wont starve of air. the power band wont drop off early due to turbo. that's more dependant on the cam profiles you choose. if anything, you'll reach max boost real early, and it'll just spin max boost for the rest of the power band. it might be all your stock motor needs to max out. either way, once it reaches full boost, that's it, your at full boost, the power will increase as per the motor's natural way. just because boost pressure is not rising, doesnt mean the powerband will fall on it's face.
i think the max psi for a typical T25 is like 25 or 28psi. that's more than i think the stock motor can handle (taking turbo size into account too). after full boost (if it's like 8psi for say), it's the wastegate's job to let any extra pressure escape out of the turbo system, and keep the boost at w/e level you choose. if max pressure is 28psi, and you have it set for 8psi, then 20psi will be blown off, or escaping the pressure system to keep it where it should be.