Is there a better measure of undergraduate academic performance than GPA?
I believe that there cannot be any single numerical statistic or small set of such statistics that summarizes the nature of a student well.

The first reason for my assessment is that there are too many different ways one may wish to use information about a student, and too many ways that a student can have an unusual background that will not fit into those boxes well.  The second reason is that any metric, once established, can (and will) be optimized against, thereby undermining its value.

Summarizing a student on one or a few metrics is no better than the much-bemoaned practices of judging faculty for tenure by a few metrics.