some say adding more mass increases sustain while some say lesser improves clarity. Fat finger according to Satch works. could this be the reason why kossov, page, clapton switched from stock kluson tuners to heavier grovers?
i have tried large fender headstock vs a smaller one but didn't notice any difference (or i may be deaf)
That IS the popular theory. Which is true at some point--but it doesn't account the wood density and multitudes of other properties of each neck has, design of the metal parts and coupling, and (most especially) what you are trying to achieve tonally.
It's hard to A-B two necks because wood is a very inconsistent material to test sonic properties on.
Here's an idea, get a cheap guitar with a 70's headstock, record clips, then reshape the headstock to a 60's shape the re-record. Easy peasy?