Is it true that the best performing employees are people who used to have a top GPA in college?
As for the long answer: there are probably libraries full of books about this. You can read some of them. GPA (or any kind of grades) are certainly an indicator, but you cannot reliably predict anything from them.
I have personally witnessed the top performing employee in a company never having been to college. Go figure.
No, not necessarily.
Here is a formal study that finds minimal correlation between GPA and adult success. To quote in part,
The average correlation between grade average and a composite success criterion was .18, a small effect. Correlations between grade average and eight other criteria of adult achievement were also small, ranging from .09 to .20. Correlational effects were larger in military settings and for studies conducted prior to 1950. The results of this meta-analysis may be somewhat discouraging to those who place a great deal of importance on the predictive value of grades.
Companies who are performing analytics are starting to find the same thing - Google, for example, is starting to discount GPA as a hiring factor once someone's two years out of school as described in this Atlantic Monthly article.
I spoke with managers at a lot of companies who are using advanced analytics to reevaluate and reshape their hiring, and nearly all of them told me that their research is leading them toward pools of candidates who didn’t attend college—for tech jobs, for high-end sales positions, for some managerial roles.
This will of course differ by the type of job in question (e.g. higher education and the military), and to a degree since some organizations select for college/GPA there is a self-fulfilling correlation to success in those organizations.