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The wonders of quoting out of context:
as long as you can get fairly aero, you don't need to be any more flexible.
So I'm not saying don't be flexible, I'm saying be no more flexible than you need to get into a tuck.
As for distance, the further, the more of an effect it'll have. It's marathon runners that I first heard it about.
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You're missing the point. If your flexible enough to ride comfortably in position x - why would being a bit more flexible to ride say 1cm lower (or longer or whatever tests faster) than position x not be 'better'?
Also, having an aero position but having flexibility headroom that allowed you to get lower temporarily or tuck with less strain for longer will allow you to ride with less drag.
"as long as you can get fairly aero"
And how do you do that without some degree of flexibility?
Also, going fast for how long? 4min, 25mi, 100mi, 500mi?