18psi is not an issue, ive saved a few sheets for personal reference. once you run too much boost you start to choke up top. None of the graphs I have saved have any issue hitting 250, and the area under the curve is enormous. plus they go right to top revs before they dare tail off let alone redline, an excellent turbo.
as for 18psi and weak pistons, sounds like good gospel. Boost is a tangent like fuel and timing, any of the above can create nonsense combustion temps and pressure on the motor if out of proportion. Boost itself doesnt kill the motor, the tune does, seen 170000k CA eat 20psi for years to come daily on OEM bottom end. Good tune to say the least, its still running, has been a regular drift cop since 08 and been running 20psi since 2005.