Clemson my mind is trained to think and I believe the fin is just fine. Remember that I don't have slashing and competition style menuvers in mind. I want speed. The annular wing has the least Ammont of induced drag which is drag caused by the end of an aerodynamic appendage.. Turbulence which increases with speed. The tunnel does have more skin friction drag but skin friction decreases with speed while induced drag increases with speed. So I would think that a closed loop annular wing fin would be conducive to achieving better speed.
I'm basing these statements off research about annular wings and also reading the history of surf fin experimentation. All of which are siteable.