Safe power level is something learned by experience. One of the other boards I frequent has a sticky where people report engine failures and the associated mods, boost level, and typical use.
In general, all else being equal, a well maintained engine driven hard, but not abused, will typically deliver some 'baseline' performance level for something approaching the life of the car. OEM tunes seek to minimize failures within the warranty period. Increasing the output of the engine will typically decrease its lifespan. Friction, wear, fatigue, and increased risk of breaking the weakest link all contribute to this.
Picture a graph showing peak power on the Y axis and engine life on the X axis. The OEM tune will show up as a point near the bottom right corner. (Lowish power, longish life) Turning up the boost will result in another data point a bit higher and to the left. If we had a statistically significant number of similarly maintained engines running a broad range of tunes we could connect the dots and have a pretty good idea of what an N54 can handle.
I suspect that most engines give up only a little life until engine loads (proportional to boost, power, torque) exceed a threshold we'll call 'safe' at which time life expectancy drops dramatically. I think that's what everyone would like to know.
As you can probably appreciate, this is not something done with a calculator or a spreadsheet. It requires experience and a bunch of interpretation. For example, if someone running 15 psi of boost advances their ignition a bit too much and the knock sensors don't tell the ECU about it (it happens!) and detonation knocks the ring lands off - is 15 psi 'unsafe'?
I'd love to see a sticky where members list their mods, tune, driving style (drag, circuit, spirited street driving etc) mileage at particular tune levels, and any problems they've experienced.