I do use grade 8 steel to make breech plugs from, and i agree it is good and strong. Actually i would rather drill grade 8 steel, than the 18-8 stainless, but i haven't been able to find 10-32 flat head socket screws in grade 8. I am kinda forced to use the stainless screws by what's available. Drilling the flash hole in a grade 8 screw would be so much easier, than it is in the stainless.
What i posted about Yield strength and tensile strength shows that the 18-8 screws are at least 3 times stronger than they need to be, and perhaps even more. Even if grade 8 would be stronger the 18-8 is a way plenty strong enough, and it may be better than grade 8 in resisting erosion of the flash hole.