Although a larger pressure tank with a low air pre-charge will give a few gallons extra during peak demands, it won't help but for a minute or so. The 5SQ pump will most likely handle the job fine, I just don't like to see dips in pressure because of a little extra demand. I would still use the 10SQ-380, and not have any dips in pressure as it will supply plenty of water.
A 5 GPM Dole valve would limit the output of the pump to 5 GPM. This would keep you from ever pumping the well dry, but won't let you have anymore than 5 GPM for peak demands. With the pump set at 400' and the static at 250', you have 225 gallons stored in the well to use as fast as you need. A 10 GPM pump can supply 10 GPM for up to 20 minutes before the well runs dry. This can handle peak demands better than a larger pressure tank with a low air charge. The SQ has a built in control box with dry well protection. So, if you do run 10 GPM for more than 20 minutes the pump will shut off. I prefer the way a Cycle Sensor protects the pump from running dry, but the built in feature of the SQ works fairly well.
One of the good things about controlling it with a Cycle Stop Valve is that the CSV works like a variable Dole valve. As long as you don't use more than 5 GPM, the CSV will act like a 5 GPM Dole valve and not let the well be pumped dry or destroy the pump. But when you need more than 5 GPM for peak demands, the CSV will give you as much water as you are using, so there are no dips in pressure.