250F - 280F - ouch . . . . all due respect, but I suggest that is bad advice.
You're engine wont last long at those temps . . . (it's going to wear out - lose compression)
If you need to run those temps to get decent performance out of your engine, there is either a mechanical issue with your car or motor, or a problem with the accuracy of your temp metering device.
I tune my race engines by ear and feel - leaning from rich until I get clear RPMs, no run-on after a wot run, and my plugs look good . . . that changes from engine to engine, whether I am indoor or outdoor, and the ambient air temp.
Normally, when I have my 'race tune' in the engine I'm running around 95-110c (205-230f) . . . if my temps were 250f+ I would be concerned about my plug, and would be putting more fuel into the engine.
One might think that their engine sounds great zinging away at wot on a lean tune, but in reality, you are harming the engine and hindering the performance.
Air doesn't make power - fuel makes power.