>Seems like the obvious outcome would be a lot more students who struggle, necessitating a slower track for them.

That is not obvious to me. It is a known psychological phenomenon that children will meet external expectations, high or low. In sports it's commonly referred to as playing up or down to your opponent. Other countries have much higher standards and have not seen an explosion in children struggling. What is most likely to happen is... nothing. The percentage of kids exceeding and struggling will stay largely the same.