Ok let me rephrase that, psi is not the only thing to consider when making power. They may still be making power by 15psi, but run a stock turbo at 40psi and see how much more power you are creating. They simply don't flow enough. Flow is starting to run out by 14psi, and you won't see as much of a power increase anymore, which is why it is not worth the risk of blowing your turbo. The increase of power from 7psi to 10psi is greater than the increase of power from 14psi to 17psi. If psi is what produced power, then even a stock turbo run at 40psi should be making insane power, even if it blows up in the process.
Don't get me wrong, psi does obviously have something to do with power output, i'm just saying that it isn't the only thing to take into account. A car run at 15psi will not necessarily beat a car running 10psi.