That results graph is from tests on firmware .164 not .188 but even so it's 5GHz range is superb.
Lets hope Asus updates the Broadcom driver to the new verion 6.x on RT-N66U also.
Broadcom would have to actually optimize it for the older chipsets. As you have seen, that's currently not the case.
BTW, Asus are usually those using the newest driver releases from Broadcom. Last time I checked, Asus's driver was several revisions ahead of those used by DLink and Netgear in their own 802.11ac routers.