I expect you already did extensive reading on the research regarding outcomes for grade acceleration, so I won't cite any of those studies. Presumably, this is a well-thought-out decision, in which you clearly involved your DC and all other relevant stake-holders. So all that being taken as read, I'm going to comment mainly on those items not related to your research, planning, discussion, and thoughtful analysis.
Redshirting threads: it might be worth considering that people who post in these threads either are self-selected for successful redshirting experiences, or have a vested interest in portraying them as successful (since culturally, we have a conflicted perception of students who are old-for-grade), in order to justify an historical decision that can no longer be reversed. Since the research on retention and redshirting of students who have not missed extensive formal schooling (say due to illness, being a displaced person, war, etc.) is uniformly and consistently neutral to negative on outcomes (with the possible exception of some athletes), it is particularly important that parents who have chosen to hold out students for non-evidence-based reasons maintain a strong positive perception of that decision, since that is the primary protective factor for the child, against negative outcomes.
IOW, once you've made the decision to redshirt a child for non-evidence-based reasons, it becomes incumbent on you as the child's caregiver to cast the decision in the most optimistic light possible, in order to fend off the negative outcomes that might otherwise accrue due to your decision. Hence, glowing redshirting threads.
Other smart kids: Firstly, each family makes the decisions that are best for their individual children, in the context of their family values and constraints. You are making the best decision you can, in your own family context. And secondly, to put it in perspective, kids who score in the GT range constitute about 2% of the general population (1 out of every 50). Yours scores much higher in that range (1 out of greater than 1000). So maybe the decisions the other families you know work for them, but it doesn't mean they should work for your child and your family; they may not have the same needs your child has.
