Why must you always treat everyone as if they were of inferior intellect?
That is extremely elementary physics and flow dynamics, and I'm sure everyone realises that boost is just a measure of engine resistance to airflow.
But that is the accepted way of measuring what the turbos are doing, as not everyone is fluent in CFM of airflow or RPM of turbo shaft speed
Yes, not all psi are equal, but that's what we have to work with, and most engines in question are of stock heads and just exhaust upgrades and perhaps camshafts in the way of airflow upgrades.... Boost is a good general indication of turbo response, all other things being equal.