Overheating does not cause burnt valves -- not in the A-series engines -- so no worries.

Burnt valves usually are caused by overtight valves, or by carboned valves -- the engine starts running rough and rather than fix it the owner continues to drive it, burning the valves.

On the other hand, when my radiator cracked on the freeway (100 kph) last September it overheated and within about 90 seconds warped the cylinder head. It wouldn't start and when I removed the plugs, water came out.