I just updated my router and yes, it appears to fix the 5GHz link rate issue. Link rate reported is 866Mbps like it was before.
I did have to manually shutdown and restart the router after the upgrade. I usually do this anyway just as a precaution. The whole startup process takes about 2 - 2 1/2 minutes before the 5GHz radio comes online and I'm able to connect which is neither faster nor slower than before. I personally don't mind the wait as long as the router works as it should but I can understand the frustration if you reboot a lot.
In the future I will be much more careful to update when ASUS posts new fw - I'll probably wait a few days until users have posted here that the fw isn't causing massive problems
Anyway, thx to house for posting the link
Props to this great community!