Thanks RMerlin and joegreat. This is something I tried, I also played with other settings such as wireless mode but nothing helped, all my devices such as the Dell XPS 13 or Nexus 9 only connected at 173 Mpbs on the 5 Ghz WiFi. Just wondering, do you also have an A2 HW version? Maybe Asus changed something and I need to wait until there is a new version based on the latest codebase...Works for me, with both an Intel 7265AC-equipped laptop and a Nexus 9, both are currently linking at 866 Mbps, being roughly 3 meters away from the router.
Try limiting Modulation Scheme to MCS9 instead of MCS11, disabling TurboQAM support. Some devices don't like it.