use search for more info on egt tuning
placement of egt changes the range a bit but, to answer ur question partially, ur making peak power, at a peak temp, becuase when ur lean, and slow, the internals get hotter, but the exhaust in the down pipe wont be heated as much as a rich one, becuase there isnt any flames coming out the exh ports, cuz it burned out already, since theres less to burn…
but for more exacting technical info on that search.
but combing that wid a narrow band o2 sensor to make sure ur on the right wide of the air fuel range, other wise u could be lean and not know it 
-Toby