I have always had a cap that I used for nitro, and that was 240ºF. This was the arbitrary number that I always went by and I still go by it today on the 600 and trucks I have here. 270 is getting up there and 284 is definitely getting up there. Running at those temps will shorten the life of the engine and if allowed to go any higher (can easily happen with just a change in humidity) could cause severe damage like melting the piston or damaging the cylinder wall.
But, I have also found that some engines like to be run this hot. Shorter life, yes, but that is the price of having fun lol. I used to purchase $100 engines, mod the crap out of them, run them like I stole them knowing that I would destroy them. I have had melted pistons, broken wrist pins, broken connecting rods and I even broke the crank once. But I was turning the engines faster than they were designed to be turned, running them hotter than they were supposed to be run, and doing all of this on 30% nitro fuel that would allow me to do this. It was the price of winning races back in the day lol.
As for glow plugs, you usually have 3, hot, medium and cold. The hot glow plug will have a very thick coil in the glow plug which requires a very lean mixture of fuel to keep it up to temp. As you experienced, your engine can and will run hot with this plug because it has to.
A cold plug will have the thinnest coil of them all. This will allow you to run less fuel and still keep the glow plug lit which will bring down yoru engine temps, but these plugs burn out or "pop" much easier than any other plug.
I have always run the medium plug if I'm just playing around. My go to plug is the OS #8. Fantastic plug, keeps the engine temps right where I think they should be and has a pretty good life.
Hope this helps.