Assuming the wing width and cross-section is fixed, the angle of attack will depend in part on the terminal speed. The lower the speed, more nose-down attitude (higher drag with higher downforce) helps. Each wing shape probably has a range of angles at high efficiency. Horizontal may not mean lowest effect + minimum drag if it's receiving air off a roof, back light, etc.
High wing placement increases efficiency by avoiding air disturbed by the bodywork, and also increases cantilever weight transfer.