Paul, i think this is probably about the only thing i disagree with you on. Say your MAF car is tuned for your 18psi on a 30 deg day, with the AFM maxing at 5000 rpm, if on a cold night(say 10 deg), you have a little overboost say to 20psi, there is no way the ecu can know there is additional airflow, hence leaning out, and no reduction in timing.
You would be surprised how much difference intake charge temp makes to A/F ratio's if there is no compensation, which the maxed MAF situation is, I recently tuned a subaru EA82t, without intercooler, then with intercooler, it was a map sensor ecu without the air temp sensor. It wasnt running lots of boost, so the difference in intake temps would not be enormous, but definately there. Originally tuned for 12:1 A/F ratios, when it came back the Intercooler had increased the density of the air to the point where the full load A/F ratios headed towards 14.5:1
My other concern with running a tuned maxed AFM setup is if it is tuned fro 18 psi, what happens at 16 psi. Assuming the AFM is still maxing, it will still be running the same load point as 18 psi, but with less airflow, so it will run rich as buggery.
All this is good and well for most people, as if the AFM maxes at 5000 rpm, there really isnt too much of a problem, as 99% driving will not show the problem, but something which maxes it at 3500rpm(which is entirely possible with an rb30) could easily spell disaster and poor driving characteristics IMO