The obvious solution is to simply allow gifted children to advance more quickly, which would be the natural course of things. Of course, the institutional and political hurdles are considerable.
I think have ceilings is unfortunate (better to have a much harder AP where even the smart kids who work really hard are challenged by the exams, too). Nonetheless, I would think hard about pushing someone through school faster than normal. It's great if children can get through homework fast (and correctly)--they can learn good habits about free time and have fun. They can even start learning about how to choose what work to do, how to not stress out about grades and external expectations, how to find their own meaning, how to find satisfaction, how to tell the difference between hard work and busy work, and not give up. I learned a lot of this by playing sports (sports psychology is wicked awesome character development).
Learning math versus more math just seems like squabbling over details to me. For a very small number of kids it might be the right choice. Everyone's different, though.
http://www.artofproblemsolving.com/Resources/AoPS_R_A_Calcul...
What this author writes about is what the supplementary math classes I teach to elementary-age students are all about.