In any heat engine the greater the temperature difference between input gas and exhaust gas the more work you’ll get out of the engine — all things being equal (the all-important caveat).
Yep. All of these are variables that aren’t accounted for in the chart.
I haven’t done any calculations (and I’m not likely to) myself, so I don’t know which variable is most important and which should be ignored.
Good exercise for a bored and curious engineer though.