I think it is smart of you to acknowledge in your letter that they have a lot of experience with educating a lot of children and that you would like to hear their thoughts and experiences. However, I think you go a little too far with buttering them up and that you sell yourself short regarding your knowledge of acceleration and knowledge of your son. I wouldn't be so quick to come right out and say that they have a wealth of experience with acceleration, when they really may not (and may, in fact, have biases against it not from experience but from lack of experience).
I really like the part about working as a team and continuing the collaborative relationship. And I like that you thank them right off the bat for working with you in the past and for continuing to work with you.
Also, when you ask about assessing beyond one grade level above, I would specifically ask that your DS be assessed beyond that, perhaps until he gets less than 85% (or so) on an above-level test. And maybe ask for it in all subjects in which your DS excels.
Good luck with continuing the process. I hope your letter moves things farther forward.
