There is probably nothing wrong with your factory bolts. The factory wouldn't use them if they weren't adequate. In fact there are many instances in hitch and recovery equipment where a "sufficient sized" grade 5 bolt is better than a gr8. Gr8 bolts are hard and brittle and can snap if subject to impact blows whereas a gr5 bolt will survive because it will stretch.
Tow hitches often come with gr5 for this reason. Mac trucks had a recall where there flywheel bolts were shearing off. The fix was to replace the factory installed Gr8 bolts with Gr5 bolts as the gr8 were found to be too brittle for that application.
I've found that usually the factory installs pretty good quality bolts, take trac bar and control arm bolts for example. They are metric 10.9, much better than off the shelf 8.8.
Generally it is safe to say that GR8 is better than Gr5 but be aware that its not so cut and dry and there are other factors to take into consideration.
That being said if I replace a factory bolt I make sure to use equivalent or better and for recovery stuff if I'm buying new bolts anyway I try to get GR8. Unless for factory bolts are rusted or otherwise damaged I would reuse them. Also once that winch plate is on it ties both sides together and the whole thing will be stronger.