You're missing an important point: there is a built-in ceiling to how good a child can be in a particular math class. There is only a finite amount of material

You're missing an important point: there is a built-in ceiling to how good a child can be in a particular math class. There is only a finite amount of material taught and a finite amount of time. So any program like this is inevitably going to greatly reduce the variance as everyone gets pushed towards this ceiling. This isn't keeping gifted students below their potential. It's just not giving them special treatment by accelerating the class material for their sake. Which is the same as before.