The mossie's not a great zoomer.. It does about as well as a P47, without the good very high speed retention. It bleeds E very quick under maneuvers.
Anaxagoras, the point of ENY is to gauge lethality and how hard it is to kill / survive plane X. Gliding around off power is very far down the list of useful attributes for that criteria.
I don't see what you mean with the TAS bit.
I mean, HTC sees the high-alt TAS of the 152, 47N and Spitfire XIV as having a role to play in ENY, even though none of these aircraft reach that potential in the arena.
As for energy retention, the power-off glide is the only way to isolate it for measurement. The pure energy retention of an aircraft is always at work in conjunction with other factors during powered flight.
What most here call "energy retention" is more like low induced-drag or aspect ratio, the factors that determine the ability to make a high G turn with minimal speed bleed-off.
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Edit: I just thought of a good example of how important pure energy retention can be: the BoB scenarios we run and the preeminence of the 110C. Even though the Spitfire is faster on paper, RAF pilots complain constantly about not being able to catch the 110C. The reason for that is because the 110C holds dive speed after leveling off for far longer than the Spitfire Mk I.