Regarding the Yamahas. The SG2000 had the same bridge as the rest of the SG range, except for the SG200 and SG3000 which had copies of the ABR-1.The Yamaha has neck-through construction of a mahogany neck and body wings, has a heavy and dense bridge, and the bridge itself is mounted on a 3/4 pound chunk of brass that's screwed into the dense body...
All that was proven back in the '70's.
The brass "sustain plate" doesn't weigh 3/4 of a pound. It covers an area the same size as the bridge unit and is less than a quarter inch thick.
The through neck is a maple centre section with mahogany either side.
I have owned, since '84, both an '82 SG2000S and an '84 SG1000S.
The SG1000S has a thinner body, slightly thinner neck profile, set mahogany neck, rosewood board (as opposed to ebony on the SG2000) and the bridge posts are screwed direct in to the top, like on an ABR-1 equipped LP.
The sustain of the two is exactly the same. The SG1000 is brighter in tone, most likely because the pickups are open coiled versions of the SG2000 pickups - they're otherwise identical.
Nothing much was "proven" in the '70s. It was mostly myths and guesswork.