The thing is, the only "stock parts" that might normally be replaced with better stuff was the pushrods, rockers, and shafts. And from experience, they gave up some power by not at least upgrading the shafts and pushrods. The biggest "downer" of building the 318 is the pistons and you notice they changed them t a performance design. The rest of the engine is fantastic.
I'll also add that the machining quality is soooo crutial. A fantastically machined engine using factory stock parts I'd venture to guess will make about 15% more power than original assembly from the advances in quality and technology there. Things like wall finish, valve sealing, reduced harmonics from better balancing, blueprinting accuracy, etc all make for solid gains using mild parts. I've seen first hand a 2bbl 360 circle track engine gain 40hp just by changing the cylinder wall finish. And it was already at 450hp on that dyno. Same dyno, same ring gap, same ring supplier, same pistons. Nothing changed but the hone finish.