I've always read do NOT get Grade 8. Grade 8 is slightly stronger than Grade 5, but it is more brittle...meaning, it breaks instead of bends...Both are enough for the weight of a human, but if for some reason they are compromised, Grade 8 will snap off while Grade 5 is more likely to bend some before breaking.

DT, the reason they hurt your feet is because you only left 3" to climb on, no duh that would kill your feet. If you use a 7 or 8" grade 5 hex head bolt (not lag screws) and drill out the tree 3-3.5", you will be just fine, and they are just as comfortable as tree steps. I'd feel safer with hex head bolts because they are 3" into the tree, I know my screw in steps don't get anywhere close to that. Get a 3/8" treehopper or auger bit, and use a cordless drill to drill out holes. You can be 20 ft up in less than 10 minutes easy, and no PITA screwing in steps nonsense...drill out a 3/8 by 3-3.5" hole, slide in the bolt, and climb. You can remove them with you on the way down to keep your stands from being stolen, or just leave em in all year.

While treesteps are meant for this, I'd rather stand on a 3/8 bolt 3 inches into the tree than a screw tipped treestep MAYBE an inch into the tree, if that. I hate treesteps, and am going to try to switch all over to the bolts this year if it works out smoothly...Unless you fork up for the expensive ones, one or two trees into a step and it is toast, dull as a butter knife. Bolts are cheaper as well.