Karl,
Regarding the SPA set up then, could the 38psi just be a massive spike that the gauge has captured or does it REALLY hold this boost as you say ?

I've seen with my own eyes the diminishing returns of running too much boost and you KNOW this to be true as well . Mind you, I bet one of your intercoolers would sort it .

However, I would concur with your "guesstimate and say 520bhp-530bhp... .