Like its been said a million times before, its not boost pressure you need to worry about, its the power it makes on that boost pressure, AFR's and knock. Boost pressure on its own means jack when it comes to "safe pressure". 20psi on a T25 is a lot different to 20psi on a T04Z, its relative to the amount of airflow it is pushing and therfore the corresponding power it makes. Run it on 20psi, see what power it makes on the dyno, watch the AFR's and make sure its not running dangerously lean, make sure your fuel system isn't struggling to keep up and make sure it doesn't knock. Simple as that. Knock, heat and excessive rev's kill a motor, not boost pressure.
End of rant.