Jacopo,

Long ago (in the '70s) when the handicapping movement started (thanks Carl Herold) One of the metrics that was used was the -6kt sink speed. That is, at what airspeed did the polar have a 6KT sinkrate. If you look at the change in the polars over the last 40 years (I've been doing it a looong time, you'll see less of a best L/D change and a much greater speed where the -6 occurs. The polar's shape has been "flattening". Small tweeks in drag at the high end produce great results, while not affecting the best L/D much.. As most XC pilots know, it's the high-speed L/D that makes the difference.