Have you looked at the book Developing Math Talent by Susan Assouline? I just picked it up to help with advocacy for our son. One of the myths she presents in the first chapter is how mathematically talented kids can be like this, advanced in concepts but not so good at computations. I'm finding the book quite helpful in giving me lots of ideas to present to the school, as well as a source to site regarding info on GT kids and math education. Maybe it would help you as well.