The firmware I am currently running - "1312-B10A-from-zyxel-jumboframes-3925adsl_phy" - is very unstable and my modem has dropped twice in 24 hours. The Internet feels less responsive with this firmware and less reliable, than with my Huawei HG612 which would stay up for weeks!
That firmware uses the
x1 adsl_phy, so I would be surprised if the
x6 one offered improvement, but excited if it did.
Another thing to try would be a much older firmware with different adsl_phy and kernel module. Will look up what the other user I mentioned found to work.
As for the perception of "slower" internet with the zyxel, I can not suggest any reason this would exist. Its possible, but I have no idea what would cause it. Did you ever sort out how to set the ports on your archer correctly under OpenWRT to get 1500 byte MTU?