I use 1650 only when I need the camera and the lens pocketable, or I need wider than 18 mm, or at gatherings where I don't want to draw attention. In these situations with pocketable, or attention I put 1650 on A5100. I don't care about autofocus improvement of 16-50 as I use the 18-135 as my every day lens. I updated 1650 without questions. If Sony says that it's better I take it. A test that would show the difference in autofocus, before and after, for both speed and accuracy would require specialized equipment and time.
It's also possible that the lens being for 10 years on the market, it needed an update to keep up with A6100, A6400, 6600 autofocus capabilities. Sony could not update to v2 just the current production without users screaming for update on the already sold lenses. It's also possible that lens update is effective on the newer cameras with fast autofocus, but not much difference on older cameras released in 2014-2016, thinking of A5100, A6000. But, if it makes a difference for the better, even a little bit, why not? To quantify the difference? Good luck.