The only theory I can support is the sanding of fins to minimize spin out. Faster or slower I can't say, but there does seem to be better control with sanded fins (formula and slalom).
When fins do get dinged or scraped, I sand to remove the abrasions. A half polished, half sanded fin doesn't sound like a good thing, so if my fins come unsanded (rarely), then they get sanded before use.
Regarding boards, I sail them the way the come, some polished, some with very fine sanding. I can't tell any difference.
What do the America's Cup boats do. If anyone has the money to research the fastest hulls, it's them. Polished? Sanded?