When I ran nitro touring using .12 novarossi motors, our temperature goals were 260* after 2 minutes and 280* at the end of a tank (starting to go lean). At times, we would see the engines approach or exceed 300* when they weren't quite right. Never really had issues with the engines running this way.

That being said, I usually am never able to get a .21 above 250 without a noticeable lean bog that seriously affects performance. If I come off in the 240-250 range, I'm happy (although others will think that's too high). I really only use a temp gauge after mains and/or to illustrate to people a proper running engine is more important than temperature.