Low speed wind tunnels are interesting, and you can come up with some drag coefficients and other numbers that may or may not tell you much. I have also had the opportunity to listen to the airplane folks at EAA talk about this sort of thing at length. Aerodynamic shapes are better, and you need to keep that in mind when you build things.
Rounded edges are good, airfoils are better, but how much is hard to say.
The real problem is surface friction. Your body presents more irregular surface area than anything else in the boat. This is the reason that many of the top racers wear skin tight body suits when they are sailing. In the end, I would spend my time making sure the plank doesn't twist the runners out of alignment when it flexes. A plank that twists the runners out of alignment is likely have a greater adverse effect on speed than a plank with square edges.