iam running a mesh with 3 R510 AP with Unleashed Firmware 220.127.116.11.195.
There should be an AP-Group Option for configuring 11n only Mode.
But there is no such Option. This one is missing.
I believe the '11n only mode' feature has returned to the GUI in Unleashed version 200.12. The screenshot on page 267 of the 200.12 UG shows the feature in the AP group config, as prior to 200.11.
I hope that helps,