If the hull is rough, damaged or badly worn, sanding with 1200 grit wet n dry will improve speeds. Just be sure to follow the existing shape/contours.
if you are just looking to sand out any imperfections, I'd be pre-checking things with a decent spirit level to see if it actually needs doing. It's really only worth sanding the section of the board which touches the water, front footstrap to the tail.
Fin drag plays a much greater role, also obviously sail tuning. A baggy sail will lift the board out of the water faster but a more downhauled sail will have less drag and more efficient traction at speeds.
Aren't most boards already like this when you buy them new? I know both my Tabou and JP boards have a non-gloss finish on the bottom. No idea what technique is used in the factory to achieve it, but you can certainly feel and see the very fine 'sanded' surface underneath the board, quite different to the glossy rails.
Formula racers used to wet-sand their boards with 1200, some with 600 grit. Some boards came that way when new, such as the later F2.