You're forgetting the role of money in completion statistics. A lot of people drop out from college because they are working to make it while in college and if they don't have deep pockets to support them, which most African American students lack, then they will not finish. Also, a lot of scholarships like the HEOP, which I recall being a common scholarship in NYC, will drop you if you fail a course or get below a certain average. So, students on such scholarships, which are minority students, will drop out under those circumstances. But, as I was saying, the #'s say that 1) GPA better predicts college grades than SAT/ACT and 2) minorities and women get better college GPAs than their SAT/ACT's predict, whereas white men do worse than those standardized tests predict.
As for rojo's example of the 660M would-be engineering major with 4.0, the grades on average do a better job predicting, but at the very high levels that may not be enough. There are thousands with 4.0, but they don't all have the ability to do engineering at Cornell. That's why the achievement tests and national/international math competitions, as well as which hs/college courses they've taken, help to winnow down the competitors there. I know that on a graduate admissions committee I saw lots of people with very high GPA's and the GRE's were up and down, but what was really telling were the essays in the subject matter. Some with 3.9/4.0 couldn't write at all, whereas some with closer to 3.0, could write an outstanding paper and was absolutely ready for graduate school. I ranked that student very highly and convinced others to do so. So, this can be subject-specific.