The predicted downforce neglects the fact that the wing material (ABS?) is too soft, so it flexes under load. I would expect this to "absorb" a lot of the predicted downforce. Plus the visible vibration of the wing seen in the rear view mirror is annoying.
Notice in the WRC picture the wing is totally different from the street STi wing. The STi wing is almost oval in cross section (not very wing shaped), while the WRC wing has a strong, high downforce wing shape (sort of bannana shaped cross section).