No formal research, just our personal experience. DS's school does cluster grouping from 2nd thru 5th. It works very well for the students and the teachers for the most part.
The only concerns I have heard are from parents with HG+ kids. They have seen schools that say "well we cluster grouped them what more do you want?" without recognizing that a child may still be two to five years ahead of the cluster.
We have been lucky that DS's school agreed to a mid-year skip from 1st to 2nd and into the cluster. So he gained a full two academic years by making the move. It won't be enough for long but it'll likely be solid for another year. Then we'll have to work with the teacher.
