I don't think you're quite getting the effect of headwork and 280 degree camshafts though
At the risk of treating you like a n00b (which obviously I know you're not), you have to remember that boost is just a measure of head restriction, NOT airflow.
Lets say that particular turbo kit needed to push 1200CFM of air through the motor to hit 417rwkw....
So if you strapped that entire turbo kit and accessories onto a stock motor, you will need to push 1200CFM of air through the engine to make the required power. This might equate to a 40psi (arbitrary value) head restriction to make that power.
But because this engine has had work to reduce the head restriction (and therefore improve the airflow through it), although the turbo is still pushing 1200CFM of air through the motor, you're only measuring a 23psi head restriction (commonly called "boost")...
"hence would need plenty of boost to make the sort of airflow for over 400rwkws...."
That doesn't really make sense... boost has nothing to do with airflow... It has everything to do with head restriction... You can have eleventy billion CFM of airflow and 1psi of "boost" if your head is good enough
Sorry if that's bleedingly obvious to you, I just wanted to make sure we're all on the same page here