ARP head and main studs are not Grade 8 - lets start there. Their tensile strength is roughly 190,000 PSI (depending on the application and material) - Grade 8 is around 150,000. But, there is a
lot more to it than just the grade - consider material, material sources, manufacturing processes, thread design, heat treating, etc.. ARP studs have rolled threads - you won't find those when you go to Tractor Supply and buy Grade 8 bolts.
By no means am I saying that every flathead needs ARP hardware (I am not in Sales!), but there are many differences between regular Grade 8 bolts (and who knows where they are manufactured, what type of materials, what processes, etc), vs ARP hardware.
Here are a couple links in case you're bored:
https://arp-bolts.com/p/technical.php
https://www.hotrod.com/articles/why-use-arp-bolts/