I'll echo what Quantum says-- it's not necessarily the case that a single grade acceleration would make much difference on the cognitive front in terms of more appropriate instruction-- but I'll also reaffirm your concerns.
There is a BIG shift toward much more emotionally intense and distressing themes in middle school-- and while 5-into-6 might not take your child there THIS year, it would take your child there next year a year early.
Developmentally, that is something I'd be quite concerned about. That's completely aside from the drama that accompanies middle school outside of the strict curricular aspects, even.
DD managed that 3y acceleration only because she was there developmentally in most of the ways that mattered most in the social sciences and literature. If she hadn't been, oh my goodness, how much more difficult it is to accomplish appropriate instruction-- because bumping the LEVEL of instruction up also bumps the developmental level, and that just isn't right for a lot of HG+ elementary children.
Hopefully you can work something out.
