I think there's a lot of factors in the temps one runs at.....gearing, engine, fuel, glow-plug, etc. But the biggest factor for me is the type of driving I do. I do long runs where the engine is working hard and that heats it up quite a bit.

I find most of my engines run well anywhere from 240 to 260 while still being on the rich side of a perfect tune. I have had one engine (that I was experimenting with) go well over 300F many times, and hit 400F once, all with no ill effects.

I would say that as long as you have enough lubrication, the temps won't hurt it....unless of course you get way too hot and you get detonation or pre-ignition. So check for obvious smoke at WOT (a good thing)

If your engine leans out near the end of the tank, I would suggest that if a perfect tune would cause it to go over 300F, you might want to richen it so that when it runs out of fuel, it stays below 300F.

Also be wary of running a too rich LSN and over-compensating with a lean HSN. Everything looks good, but if you can drive it at WOT long enough, you will see the smoke trail vanish as the extra fuel that loaded up in the crank during idle finishes burning off. Not good.