Hi
This is pretty complex question, but I think that you mentioned most important points. Generally, engine is on its highest load during top speed runs. To prevent problems during prolonged top speed run you must be sure about oil and water temp, have good knock detection and EGT measurement. Overheating of engine will show increased values of oil and water temp, and probably in EGT. If you will do just top speed test, using at least 1 grade colder plugs like usual (just for test) will be benefit.
So for this purpose is oil and water cooling vital and will determine time of top speed run.
My opinion is that if you have proper tuned engine, values of oil temp,water temp,EGT (and knock as result) will show you time to let off gas...
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