JG Engine Dynamics in Alhambra CA did some dyno tests on BONE STOCK 280ZXT's in the late 80's. Without exception, the cars stopped making HP after 10psi. You could turn up the boost further, but it didn't make any more HP.
So Tysons claim of 10psi being the limit for a STOCK system is reinforced by Javier's testing done on his in-shop Clayton dyno in 89, 90, and 91.
Group Z did a dyno day there in 9X (90 something...) and most of the ZXT's were running 10psi, when they turned it up more, no more HP resulted, so "what was the point"...
That RRFPR really helps when you start going over 10psi. The ECU just isn't mapped beyond there, since the stock boost limiting relief valve usually pops off around 9-10 psi. If you have a limiting device installed, why map the ECU higher?
Tweak this, tweak that, all bets are off!
Then again, a Megasquirt...