I think billswift's point was that the SAT is a better predictor as opposed to using human judgement as in reading the application letter, etc... It seems plausible that adding another "objective" measure like the HSGPA will further increase accuracy. But the big point is that using human judgement will DECREASE accuracy. The really interesting paper is the following as suggested by Alicorn:

Michael Bishop, "In Praise of Epistemic Irresponsibility: How Lazy And Ignorant Can You Be?"