Grade 5 3/8" is fine in terms of strength, as you say--overkill.
I don't like grade 8 on bumper mounting bolts, because I like the "softer" grade 5 bolts that will deform in the event of an accident. I'd rather replace some bolts then bend a frame.
Worked like a charm when my little truck got rear ended by a big old Tahoe. Bent the bumper a bit, deformed the bolts, but that's it. The frame survived intact.