I think the focus on % loss is a little misguided. For any AWE, it is quite likely that zero elevation angle is plain impossible.
Conparing with a HAWT (regular windmill) yes you have losses, but you have benefits as well.
Cosine loss only means that you need more wing area to achieve the same power. This is bad, but other factors may be limiting the scale also, like tether strength, power rating of the ground eqipment etc.
And, for a superturbine type og windmill, if you point the rotors horizontally, you need to add an extra force to counter tether downwards pull.
Also remember higher elevation would mean more altitude and higher average windspeeds.
Im not sure cosine losses is the best place to start a design…