Ok, so lets pick >your< best current level - doing any kind of ride... TT, hill climb, rolling loop...
And you want to 'improve'...
you accept that the studies above are accurate an applicable to you... and that the most 'efficient' cadence is somewhere around 60 rpm ish....
how are YOU going to 'improve' ???
there really are only 2 things, improve power and improve efficiency - efficiency comes from 3 factors, aero, mechanical, and 'motor' - mechanical is very minimal which includes all aspects of the 'machine/bike' (tires, drivetrain, overall weight, etc...) , aero - again somewhat equipment, somewhat ride/position, a lot of the environment (flat road vs steep hill, head vs tail wind, etc...)
...finally - the efficiency of the 'engine/motor' - can this be improved? - let's leave that for the moment...
Back to improve engine/motor 'POWER' - if you accept 'efficiency is already predefined at somewhere round 60 ish rpm - what and how do you improve the only other option open, your power?
will it be enough? you're already 35-40 yrs old (or older...) , and not as 'powerful as you were 10 yrs ago... you have many constraints on your time/ability to maximize your 'power improvement' - what's left to do to improve?
here's a great video on results of doing ONLY power based work for a period of time and the result of it, eliminating any possible change in 'motor efficiency'
https://www.youtube.com/watch?v=J4UJVeO7_eg - whoop commercial aside...
increased power enough to 'improve' actual riding over time?
... back to 'efficiency'
if you accept that 'efficiency' is within the studies' observed bounds, can there be substantial/substantive efficiency improvement? or are you limited?
OR
is YOUR engine/motor efficiency not yet at it's optimum? (not observed in ANY of the studies...)
if there's an opportunity to 'improve' your efficiency, what can/will you do to improve or define your 'efficiency' and it's limits?
Ride On
Yuri