sustained high speed running might be considered for using the engine for racing, a lot of water skiing, etc…
Evidence of an engine that is used extensively at high speeds might be excessive electrode wear.
Don’t know about the 5 minute time, that alone would not be considered sustained high speeds. It is more a function of the percentage of total run time that is spent at WOT, so I am thinking a percentage of at least 60% might be considered sustained high speed use….