Why hydrofoil wing span has any relevance to the turn radius? It is not a kite which span measures in meters, and has some non-negligeable moment of inertia. It is attached to the board which has more significant impact from that perspective. And, even with the board consideration I refuse to believe that the length of the board is of any significance during the turning maneuver.
A kite hydrofoil 360 can be idealized as a mass tethered to a point on a circle trajectory. The relevant variables are:
1. The mass (mostly the rider)
2. The force of the pull (kite + foil)
3. The speed
Given that the variable 1 is pretty much fixed, I suggest that one obvious way to have a tighter turn is decreasing the speed. The other, less trivial possibility is is increasing the pull from the kite or push from the foil.