Re speed ratings for tires. I do know from experience, if you run 40 mph on a tire that is getting low, like maybe below 20 psi, it heats the tire a LOT. Mine melted from the inside, looked fine outside, but when I took the tire off it had melted on the inside, looking like a bowl of spaghetti got in there. Cord all over. That took about 20 min of run time, but it was during a race, so you might get longer if you are not cornering so much.
You should have no problems with 30 psi, but my experience has been that there is a huge handling difference between 30 mph cruise, and 40 mph. That's frame related more than tires.
Anyway, if you inflate to the psi they intended for fat tires, don't run 40 mph for long. If you want to run fast on a fat tire, get motorcycle or moped rims and tires, that are wide, durable, and intended for fast speeds. Many have done that here. That is how you do a fast, street, electric bike best.