bk,
Remember the shape of that belle, the bell curve. It's really big and fat in the middle. The top quartile could, I am sure, do well with an enriched or accelerated math, but it isn't likely to be any where near what your child needs. OTOH, a group like this a few years older than your child might fit the bill nicely.
there is some work by Cheryl Fox with the Michigan Multiage Continous progress Model and McClellan and Kinsey's Grouping for learning that says that everyone learns more if they are near their "readiness level." As to how much more I think that would depend on if the teachers are willing to pretest and compact.
Of course the big demonstration of how fast kids can learn math is Julian Stanly's work at John Hopkins CTY. He is the one who started giving 12 year olds the SAT as a way to look for Math Talent, and then teaching them a year of Math over one Summer. This may not relate directly to the info you are looking for, but an email to the current CTY staff might get you some answers.
Best Wishes,
Trinity
