The optimized transfer on top of ZFS basically refers to the use of send/receive with delta for snapshots as opposed to the basic rsync default behavior.
The migration API doesn’t expose the availability of other instances on the target, even if that could be used to further reduce the transfer and eventual dataset/volume size. The reason for that is that the migration API is designed to work in an environment where the source and target server don’t have to trust each other and in an environment where an unprivileged user that only sees a few instances may be the one triggering said migration.
Images would be something we wouldn’t have the same security concerns about exposing (for public or cached images anyways) but for those we have a different problem. While the exact same image may be loaded on multiple servers, the downloaded image is a tarball or qcow2 which is extracted into a ZFS dataset or volume. That means each server will have independently created the same dataset or volume. While the content is basically guaranteed to be bit for bit identical, ZFS will not have assigned them the same ID and we therefore can’t use them as a send/receive parent dataset/volume.