Re: bridge pins
I think it’s a matter of how solid they hold the strings terminal end at the soundboard. While plastic pins hold the pins in place, I think they don’t give as good a connection with the bridge, and, thus, the soundboard. While it may all be in the mind, I think there’s a direct correlation with mandolins. Many owners change out the original stamped steel tailpiece for a beefier solid cast bronze tailpiece, and claim a better sound.
While the vibrations are focused at the saddle, I don’t think you can dismiss any part of the whole as not having some impact. Think about the difference between the sound of a slide on a guitar string, and a fretted note. They both provide a hard contact with the string to change its effective length, but there is a noticeable difference in the sound.
And, Of course, guitars painted white sound better. Right Billy Corgan?