Interesting question. As a structural engineer I figured it's something I should know, but didn't. Looked it up. Looks like most hitch pins are in the grade 5 spectrum. Grade 5 is a SAE grade (Society of Automotive Engineers) that is about 90,000-120,000psi in terms of strength. There is an equivalent ASTM grade 325 that is used in construction, e.g. buildings, bridges, etc. Same strength just different grading agency.
Grade 8 (SAE) or ASTM 354/490 is about 150,000psi. A decent increase.
A cheap lag bolt, or hardware store bolt (from China) is good for about 30,000-45,000psi to give you some reference.
Two caveats, bolt strength can vary slightly based on bolt diameter due to heat treating processes. Second, threads reduce a bolt's strength, so a 5/8" pin without threads, is stronger (more cross sectional area) than a 5/8" threaded bolt and good design practice requires no threads in the shear plane - i.e. where the bolt touches the steel of the hitch receiver.
