5 GHz Throughput vs. Attenuation
5 GHz downlink again shows the 4.372 firmware falling off faster after 21 dB than the two 3.354 runs.
ASUS RT-AC66U 5 GHz Downlink Throughput vs. Attenuation
The 5 GHz uplink plot appears to also show lower throughput from the 4.372 firmware as signal levels fall.
ASUS RT-AC66U 5 GHz Uplink Throughput vs. Attenuation
So, does all this make the RT-AC66U a less attractive buy? The Router Ranker now shows the ASUS in the #2 spot behind D-Link's DIR-868L among AC1750 class routers. But looking at the Ranker performance detail comparison below shows very close total rank scores (1.7 and 2.0) and higher wireless range ranking for the ASUS.
ASUS RT-AC66U & D-Link DIR-868L Ranker Performance Summary Comparison
Looking at the throughput vs. attenuation (profile) plots is also instructive. So I plotted the current top four ranked AC1750 routers for comparison, i.e. the D-Link DIR-868L (#1), ASUS RT-AC66U (#2), WD My Net AC1300 (#3) and NETGEAR R6300 (#4).
2.4 GHz downlink is pretty much a horse race, except for the ASUS' initial throughput dip with strong signals.
Four Top AC1750 Routers Compared - 2 4 GHz downlink
2.4 GHz uplink shows all routers evenly matched until the NETGEAR, WD and D-Link start to fall off, while the ASUS maintains higher throughput awhile longer. This indicates that the ASUS should have wider 2.4 GHz uplink coverage than the other products. But without matching downlink coverage, this uplink advantage is of questionable value.
Four Top AC1750 Routers Compared - 2.4 GHz uplink
5 GHz downlink shows the ASUS tracking above the other three products from 9 dB of attenuation on. However, performance is close enough to the other products except for the NETGEAR that you might not notice the difference in real world use.
Four Top AC1750 Routers Compared - 5 GHz downlink
The story for 5 GHz uplink is similar to downlink—the ASUS should have the best coverage of this group while the NETGEAR would have the worst.
Four Top AC1750 Routers Compared - 5 GHz uplink
It appears that NETGEAR is correct that the new firmware posted for the RT-AC66U produces lower throughput. This effect is most significant running downlink and at lower signal levels. Throughput with strong signals, doesn't appear to be affected.
I have no way of knowing whether the tweaks ASUS made in this latest firmware bring it into FCC compliance. But even with slightly reduced performance, the RT-AC66U is still a top AC1750 class router.