I am curious as to the "grade 8" aspect of the OP's question. Is it to try to avoid future thread damage? The more common reason might be to keep the axle from bending, but no mention of a bent axle is made and the photo shows a relatively straight axle. We now the axle isn't broken, the main reason to use high strength bolts (and allowing a higher tightening torque).

The many axles with thread damage I've dealt with are so from poor axle nut (or lock nut) management. Meaning the nut gets loose so it floats a bit and gets pressured along the axle's length or that the nut is over tightened and it skips a thread. What I don't know is whether a higher strength material would resist the kindof causes. Andy