Simaril: That's a little bit more tricky of a question.
As a generalization I think it's about average (maybe even just a little above avg) in energy retention. The Mustang's mystical energy retention is mainly situational. Let me explain:
It retains energy well if you fly it using low-g or very short duration maneuvers. But any aircraft would retain energy flying in this manner. What helps the Mustang in these situations is that it has a lower parasite drag coefficient compared to other planes. That's part of the reason for it's higher top speed for a modest engine horsepower. The same factor of course contributes to retaining energy because it bleeds energy slower from parasite drag in low-g, high speed maneuvers. Enter a fight when you have energy in either altitude or speed, use maneuvers sparingly, be disciplined about trading that speed for alt. Combine that with a lower parasite drag and the Mustang hangs on to it's energy longer.
However yanking the stick back for max performance turns, it bleeds E in spades especially if you hold that high-g turn for more than a moment in the horizontal or the vertical. It has a high wing-loading contributing to induced drag in turns. To top it off, it has a very modest thrust-to-weight ratio so it's like waiting for paint to dry trying to gain that energy back through the powerplant so it really sucks in energy retention in sustained maneuvering

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Tango, XO
412th FS Braunco Mustangs