On 5GHz, my link speed is stuck at 54 Mbps on my late 2013 Macbook Pro Retina with Broadcom BCM43xx 802.11 a/b/g/n/ac adapter. I can't get the 5GHz link to connect any faster, though 2.4 GHz reaches 217 Mbps.
This is true of the stock firmware, and the latest 3.0.0.4.376_2061. I tried the devices at 5 feet apart, and then 20 ft. I toggled every relevant option in the router settings. For kicks, I tried resetting my MBP's SMC and PRAM, and even blew away my SystemConfiguration. I mention all this only because I didn't see any other posts about poor 5GHZ link speeds with this router and Macbook Pros explicitly.
It's not my machine, as I confirmed it can still reach my previous router (Asus RT-AC68U) at 1GHz. I was just hoping this newer router could help me extend my range and speed a bit, especially when connecting to my link aggregated NAS or Windows box.
Hoping it was just a bad unit, I exchanged the AC87R at Best Buy. Despite getting a new unit with a different production batch code, the same poor 54 Mbps connection persists. I wish I could wait and see if future updates help, but don't want to gamble with ~$300 bucks. This device is going back to the store. If someone chimes in later with a fix confirmation, I might be tempted to try again.
P.S. Gary Key, I know of you from Anandtech, and offer you my kindest regards.