In an ideal world I totally agree we should expect most features to work as announced by Asus, but sadly this is the real world...
I also agree you should be able to use 40MHz bandwith, but it seems you would be much better off with just 20 for now...
Did not stock 384_20942 or 384_21045 improve anything for you? (on at least the one box it's available for)
How is your setup ? (router, ap or repeater mode)
Since you do not have to consider neighbours, is settings on auto, or have you handpicked wireless channels etc... Initial setup, did you choose wireless as one network, or separate networks (2,4 and 5 GHz) to see if it makes any difference?
And I also assume you've already been through all the known tweaks as described here:
https://www.snbforums.com/threads/r...-5-is-now-available.46575/page-13#post-405971 even though it should be totally unnecessary, since it should work as expected...
And just so I've mentioned it, I blame all these problems on Asus, and just pray Merlin doesn't give up his fantastic fw anytime soon...