A lot of what I have read points to wanting an accelerated kid to still be high achieving in the receiving grade for maintaining confidence. They might have to work harder to get to the top, but having challenge is the point, so I believe you want to see those scores still around 90% or so a year ahead. You wouldn't want to accelerate and then have your child at 50% -- that means they could probably be met with 1 year above differentiation in their existing grade and be high achieving.
As for gaps, since curriculum typically spirals, a lot of what they learn is touched on again in future grades, only with increasing depth and more vocabulary. The idea is, if a gap is noticed, then the teacher should take that chance to fill it in. I think the primary grades may be easier to fill curriculum gaps, as I can say skipping part of 4th-5th meant DS missed out on some content in social studies that won't all spiral back, so we are going to do some on our own.
