I also tested the EX6200's "FastLane" feature in both modes with surprising results. 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. Since I measured 47 Mbps down and 54 Mbps uplink using the straight 5 GHz repeated signal, it looks like this FastLane mode isn't the way to go.
NETGEAR EX6200 extended throughput via FastLane w/ 2.4 GHz backhaul
Switching over to using a 5 GHz backhaul looks more promising. The straight 2.4 GHz extended signal provided only 21 Mbps down and 26 Mbps up, while the FastLane extended signal yielded three times more downlink and almost two times more uplink throughput! So if you had an AC1200 or higher class router serving mostly 2.4 GHz clients, it looks like the EX6200 could put the idle 5 GHz radio to good use.
NETGEAR EX6200 extended throughput via FastLane w/ 5 GHz backhaul
So how much did the EX6200 actually boost the signal in our test setup? Glad you asked, because I took inSSIDer measurements to show just that. The 2.4 GHz plot below shows the EX6200 took an intermittent -88 dBm signal and boosted it to a continuous -43 dBm average signal.
inSSIDer plot of 2.4 GHz signal difference
The 5 GHz plot shows a similar story, with an intermittent -86 dBm signal turned into a much more reliable -41 dBm.
inSSIDer plot of 5 GHz signal difference
There is still no substitute for the reliability and throughput that Ethernet-connected access points can provide for solving wireless coverage problems. But the EX6200 shows that the higher bandwidth afforded by 802.11ac technology can be a surprisingly good substitute. And much easier to set up, too!
While you can use the EX6200 with any router or AP, you'll get the best throughput by pairing it with an AC1200 or higher class router. As with any wireless extender, the more bandwidth it receives, the more it can pass along. For the EX6200, that "extended" bandwidth can be impressively high.