Performance - more
5 GHz downlink shows cyclical variation in both plots, which means it is coming from the link between the Broadcom-based ASUS RT-AC66U reference router and the MediaTek based EX6100.
NETGEAR EX6100 throughput - 5 GHz down
5 GHz uplink shows similar cyclical variation, but with much lower average throughput.
NETGEAR EX6100 throughput - 5 GHz up
Perhaps we're seeing an incompatibility between Broadcom and MediaTek 11ac implementation. Or maybe it's just link rate hunting. At any rate, if your primary goal is repeating 5 GHz signals, you might want to err on the side of moving the EX6100 closer to the router you are extending.
I also tested the EX6100's "FastLane" feature. The first plot shows extended wireless throughput using the 2.4 GHz radio to connect back to the base router and the 5 GHz radio to connect to the extended test client. The resulting throughput is slightly less than using straight 2.4 GHz extension. You can also see the cyclical variation from the 5 GHz backhaul link.
NETGEAR EX6100 extended throughput via FastLane w/ 2.4 GHz backhaul
Switching over to using a 5 GHz backhaul isn't that great either. Downlink throughput is about the same as from straight 5 GHz extending and uplink is only slightly higher.
NETGEAR EX6100 extended throughput via FastLane w/ 5 GHz backhaul
The 5 GHz connection is the weak link in both FastLane configurations.
The problem with dual-band repeaters is that 5 GHz signals don't go as far as 2.4 GHz. So unless something is done in the design to compensate for this disparity, you end up having to favor one or the other band when you locate the extender. Our testing showed the EX6200's 5 GHz extended performance was actually better than its 2.4 GHz, so this design goal is possible. But not so for the EX6100, where 5 GHz performance is definitely its Achilles heel.
I think NETGEAR was wise to build an AP mode into the EX6100 because that may be a better use for it. There are so few dual-band wall-wart format APs available and only one (the EX6100) that supports 802.11ac. Now if they would just add HomePlug AV2 and keep the price under $100, I think NETGEAR would have a more compelling product.