I wonder about the law of diminishing returns here. While 2000+ grit would get the nails to near optical quality, the business end of the nail would dent and change rather quickly against the strings, wouldn't it? Especially on the wound strings. There's a golf analogy here, concerning "perfectly balanced" golf balls. One can take a golf ball and float it in a concentrated saltwater solution and determine the balance point, marking it with a magic marker or something and making sure that is the axis where you roll the putt, but one whack with a driver will completely change that point (never mind digging into the surface with the grooves of a nine iron). By the time you get to the first green, it won't matter a bit. I've gone all the way to something like 10000 on a sharpening steel, but my technique (or hearing) certainly is insufficient for me to discern any difference from the 1000 grit sandpaper. A few loud rest strokes against the wound basses would surely mess up all that, wouldn't it?
Hell is full of amateur musicians -- GB Shaw