Wow Dottie I didn't realized
exactly how much our kids have tracked together.
I'll just add on some of our experiences which we're still in the middle of establishing. Dottie's the trailblazer here

Our DS was also an early entrance to school so he's 7.5 in third grade. We have done math subject acceleration only so he's 7 1/2 in a sixth grade honors math class for one period a day. The rest of the day he's with his normal homeroom. We will likely not accelerate any more than the current 1 year. Like Dottie, our child is still coasting through with not a whole lot of new material. (On the drive to school he watches Algebra I & II videos on my iPhone from Khan's Academy... that's the new material for him... sometimes

)
One thing that I didn't think about and appears to be fairly common is that bullying usually is between kids near the same age. Knock on wood, but we haven't yet had any issues. And we were very surprised with how supportive the middle school's staff has been. It's been great so far and I think school support is a must.
So that's been our experience. +1 full year plus an additional +3 years subject and it's going well. Hope that helps.
JB