Ok, I'm about to sound stupid, but... Isn't the "load" on the hub and the studs and nuts are just there to keep the wheel from sliding off the hub (lateral forces only)? Look when you mount your wheels on the hub for the wheel to rest on the hub. I wouldn't trust three studs to support the weight no matter if they were 1/4", 5/16", or 3/8". I'm no engineer, so correct me if I'm wrong on where the load is on the hub.
At the same time, I don't think the 5/16" is a bad idea. I have always run 1/4" but not because I chose it, just because that's what all my karts and wheels have been set up for when I bought them. One advantage to the 5/16" studs is to get the 1/2" nuts so you can use the 1/2" socket for wheel nuts and weight bolts/nuts. I could see it being beneficial to get the gear hub to 1/2" nuts also so you wouldn't need as many different socket sizes for adjustments.
The impact warning is good. If you are putting a nut on with it just run it until it clicks a couple times and quit. Any more and you are pulling the stud apart by the threads, so you'll either ruin the threads or the stud itself. A weakened stud will fail at the worst possible time.