I am not sure to quite understand your calculation. L/D ratio (where does the apparent wind depend) is squared but the power on the onboard turbine is determined by the cubed apparent wind speed if I am right.
Let us take an example (with some approximations) from the onboard turbines: the diameter of a rotor of Makani M600 is 2.3 m. So 8 rotors lead to a rotor area of roughly 33 m². The nominal wind speed for Makani is 9 m/s. If the L/D ratio is 12, then 6 with the onboard turbines and the losses you mention, that lead to a reasonable kite (apparent wind ) speed of 54 m/s. Air density is assumed to be 1.2, power coefficient is assumed to be 0.2 (losses of efficiency due to a desired lower induction factor leading to larger rotor area; other losses because their trajectory is not rectilinear; other losses…) :
1/2 x 33 x 1.2 x 54³ x 0.2 = a little more than 600 kW.
Now with FlygenKite with L/D ratio of 2 with onboard turbines and losses as you suggest, the same “power” (in fact it is thrust, the real power is that on the wing, even if onboard turbines transmit power to their respective own generators) of 600 kW is captured by multiplying 33 by 27 = 891 m². Too much rotor area, unless I am wrong.