Well, there is no real answer to your question, because there is no limit. The key answer is that maybe you shouldn't trust that person anymore. I've used race gas countless times in my Evo, but then again I know how to use it. Without an associated retune and/or raise in boost, higher octane is just a waste of money. I always tell people to use the highest unleaded octane they can find (usually 104) so as to get the race gas benefits without any potential damage to the o2 sensor. However, there's nothing wrong with leaded race gas either (110/112/116/120). I've used leaded versions many, many times and am still on my stock o2 sensor - my car runs like a champ.
The guy that was telling you that was talking about avgas, the stuff you can get at the airport. This is what the old hotrod guys used to run back in the day before there were so many options for fuel available. The chemistry of the gas is different because a plane motor has to operate in a low oxygen environment. Iirc it is overly oxygenated and when you run it in a car, which operates in a low altitude environment the car begins to run extremely hot, this in turn can burn and sink valves which is something that used to plague the old hotrodders. Of course this doesnt tend to be the first thing that goes wrong on modern motors because of stroger materials used in the valve seats and the fact that our cars run sodium filled valves to keep them cooler. But if your running a unleaded or leaded automotive grade gasoline, no matter what the octane, you will be fine.