Overshoot is normal for thermostats. How much will depend on the quality and design of the thermostat (example: air bleed holes). As far as temperature concerns, water not under pressure will generally boil at 212F, 50/50 antifreeze at 235F, and antifreeze at 15 psi pressure (radiator cap pressure) will take 265F. Higher temps may help efficiency, but decrease power slightly.
A thermostat can usually be checked by placing it submerged in a pan of water on the stove. Use a thermometer. When the stat begins to open you will see air bubbles released. I have used this in the past to choose between different thermostats and rejected those that took too long to open.
