CoreOS stable channel current release is 1185.3.0
https://coreos.com/releases/#1185.3.0

This is the most recent stable release since 1122.3
https://stable.release.core-os.net/amd64-usr/

1122.3: kernel: 4.7.0 rkt: 1.8.0 docker: 1.10.3 etcd: 0.4.9, 2.3.2 fleet: 0.11.7 systemd: 229
1185.3: kernel: 4.7.3 rkt: 1.14.0 docker: 1.11.2 etcd: 0.4.9, 2.3.7 fleet: 0.11.8 systemd: 231

The main change appears to be the move to docker 1.11.2
https://github.com/docker/docker/releases/tag/v1.11.2

Looking at the release notes, I see no compelling reason to upgrade. The fixed bugs don't appear to be things that we're worried about.

Also, it looks like MTU handling is unchanged:
https://github.com/docker/docker/issues/22297

Since 1.9, is appears that Docker has required setting --mtu.