Page 12, from the table 2:
Flexible wing Lift-to-drag ratio 6
Rigid wing Lift-to-drag ratio 20
On this table, the Max flight circle radius is about 5 wingspans for both rigid and flexible wings. I think things are perhaps different. Also take into account the tether drag.
https://www.skybrary.aero/index.php/Radius_of_Turn :
Radius of turn is dependent on both airspeed and bank angle. The radius of turn at any given bank angle is directly proportional to the square of the airspeed. Doubling the airspeed results in a radius of turn that is four times greater while tripling the airspeed would result in a radius that is nine times greater. Conversely, if the aircraft speed remains constant, increasing the bank angle will decrease the turn radius.
As a first approximation the turning radius of a rigid wing will be higher not directly because of its higher mass, but due to its higher speed resulting from higher L/D ratio. The same when comparing flexible wings with different L/D ratios. So a good efficiency could result from an accurate balancing between a high enough L/D ratio, but not too high in order to keep a tight turning radius and gain some space.