I use the FCS system to hold the side fins on my SUP. The Slot Box appears to be a scaled-up version of the FCS concept if not a licensed alternative.
As for hydrodynamic efficiency, locating a protruding screw head directly in front of the foil's leading edge is just about the least effective way to avoid drag or screw up the flow across the fin. Locating the bolts off the the side and countersunk is a better alternative, at least in terms of flow across the fin.
My only problem with the FCS concept is the damage to the board if the fin hits a rock or something. The A and E boxes cause the fin to break before the box blows out or allows the impact to blow off the board's tail as would be the case using a Tuttle, for example.