AV gas doesn't burn "hotter" than any widely available fuel. Octane numbers refer to anti-knock qualities, which in no way translates to "hot".
Sodium filled valves are generally found on engines that continually operate at higher throttle openings, such as aircraft and marine engines. Some car engines have used sodium filled valves, and Benz is one of them. Advances in materials and much better engine management systems have made sodium filled valves, for most car engines, unneeded.