Performance - more
Once I figured out the DAP-1520's single backhaul nature, I decided to check its pseudo "FastLane" operation. The first plot below has the client connected to the extender on 5 GHz and the DAP-1520 connected back to the ASUS base router on 2.4 GHz. Downlink throughput is about half I got using 5 GHz connections for client and backhaul, but uplink is definitely improved at about 2X the 11 Mbps measured.
D-Link DAP-1520 5 GHz extended throughput w/ 2.4 GHz backhaul
Switching over to using a 5 GHz backhaul shows the highest throughput measured for downlink—about 58 Mbps—but there's that 11 Mbps again on uplink. This and the results above tell me the problem is in the 5 GHz uplink between the DAP-1520 and ASUS RT-AC66U.
D-Link DAP-1520 2.4 GHz extended throughput w/ 5 GHz backhaul
I'm glad I took another run at the DAP-1520, because my first go-around would not have yielded a very complimentary review. Upon retest and now that I understand its single link backhaul approach to "simultaneous" dual-band extension, I am more comfortable with the product.
Of the three extenders tested with the same new test process, the AC1200 class NETGEAR EX6200 is still my recommendation for an 802.11ac extender. But if you would rather pay around half the EX6200's $130 pricetag, at around $60, the DAP-1520 is worth a look.
Although you won't get the helpful placement LEDs or multi-mode support (AP and bridge) of the NETGEAR EX6100, it looks like you'll get about the same, if not a bit better throughput from the same Mediatek components in the form of the DAP-1520.