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.

yes i know what the ron number means

and ok your right, it probably doesnt actually burn hotter, but, it does burn longer, (well slower) which in turn can mean it still burning as it leaves the cyl. and i've burnt exhaust valve's on car b4 from running avgas which ran fine b4 it was on avgas, but that engine was piss weak compared to any thing merc ever built.

and as far as cars not using sodium valve's anymore

one of the best 6's ever built (along with the m103/4 ). the rb26, and nissan v8, the vh41/45 uses them aswell

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'85 190, unrego'd (prospect track car)
'87 190e, manual (kinda rare in australia)
'89 260e

If your that concerned about a possible valve burning, you should investigate what the racers use. Light weight ti or somesort of heavy duty Incconnel valve for high heat. Of course your looking at a full set of special seats.
Like I said before, it really doesn't buy you much until you turn up the timing. The USA cars came 6 degrees retarded timing. Maybe you should look through the performance archives here and set yours up for the higher timing. Of course, the Euro's had higher compression which helps you get more hp out of that fuel/timing.
I think the use of sodium valves is not just due to heat or heavy duty, but is an engineering repair for a weak design. By weak, I mean the geometry of the exhaust port, stem length etc, there just is more too much heat. Common on AV aircooled engine as I'm assuming like their street brothers run hotter.
Diesel stems are 9mm I believe where a gaser is 7mm. If you choose to purse it, check lengths/sizes... then ream the head out to take a diesel valve guide. Be aware of total valve weight with retainer as that will effect the valve float point (RPM) of the engine.

Michael