Trillium,
I think that the letter you've drafted is very effective in making your point, but then, I'm an easy sell
My only suggestion would be to drop the P.S. and add a short, specific request: "we are requesting a meeting to discuss options to meet DD's needs. We're confident that if we all put our heads together we can come up with a plan that addresses her need for different pacing and content".
You might consider including the article posted on another thread about what kids aren't learning when the work is too easy.
There's no question that there is some risk involved here. In the end it will depend on whether the district is actively hostile towards GT needs, or just clueless. Clueless is workable. Hostile is generally harder. Whatever you decide, I wish you the best!