It isnt "safer" under boost, it just makes more room for tuning error. 8.4 is ridiculously low.

Did you tell this vendor that you only plan on running 8PSI? Who is recommending 10.6?

10:1 is fine. 9.6 is fine. 9:1 is doable but youre losing power for no reason. Hes right...youre not losing a ridiculous amount, but why at all?