PSI means nothing, it is all about airflow. Eg 5psi on a T51r is going to be pushing more airflow than a stock turbo at 12psi and hence make more torque, bigger combustion pressures etc.
I'm assuming you have an rb25 if so I'd say keep the power under 300kw, tune it very conservatively (rich and easy on timing) and if maintained properly should last you ages.
Things that will kill a motor are poor tune (detonation killing pistons and bearings), oil issues eg not enough oil and drifting = oil starvation, high revs for a long time filling the head with oil, limiter bashing causing oil pump failure.
Outright torque is almost never the reason why a stock RB has failed. So to answer the question, keep pushing more boost in until you make the power you want or it stops making power (limit of the turbo) or you run into detonation problems from the intake charge being too hot. If it is making 270kw on 15psi, then I'd say it could probably go to 20-22psi fairly easily, probably be making more than 300kw then though.
Got a dyno sheet? Or know when the turbo comes on full boost? Might make it easier to figure out how much power it could make and on what boost, ultimately we will be guessing though ask ATP what they think the turbo is good for.
18-20psi should be a good limit to aim for though, just make sure you insist the tune is safe, who are you getting to tune it?