The issue with using grade 5 and grade 8 bolts is one of torque specs. You need to torque bolts to where they deform a bit in order for them to stay tight. To do that with grade 5 and especially grade 8 bolts requires that they be torqued much more.
From this chart, a 7/16-14 (which is 7/16 NC) in grade 2 takes 24 ftlbs, in grade 5 takes 37 ftlbs, and in grade 8 takes 52 ftlbs of torque to be considered torqued up. The question then is, what kind of threads are you torquing it into, and can they take that?
It's not just a simple issue of "more is better".
https://www.boltdepot.com/fastener-i...ed-Torque.aspx