I think it's a good letter and agree with Dude and Zen Scanner's points too.

You probably know this from being around here for awhile, but FWIW sometimes the easier approach is just to 'afterschool' or do outside enrichment. It shouldn't be that way, of course, but that's what worked for us...after months and months of promises about in-class acceleration. And that was at a private Montessori school, where a big part of the 'sales pitch' (especially at that school) was learning at your own pace. But I hope your approach works. Good luck!