On the contrary, the study you cite found no significant effect either way for either group. From the last page: "we find that gifted grouping does not help or hurt the
achievement growth of gifted students nor does it help or hurt the achievement
growth of non-gifted students"
(emphasis mine.) This certainly does not imply that separating gifted tracks results in a lot more struggling students.
Based on what GP said, it isn't clear that the implementation of "allowing advanced students to learn advanced topics" is successful either. It seems like the current gifted/non-gifted system isn't working.
