After nearly a week, this has been the most stable modem/router I have used apart from the HH5 Type B. Errors are lower than both the HG612 and Billion 8800NL on my 80Mb Infinity 2 connection, My Type B HH5 does report a slightly higher attainable speed and actually syncs slightly higher, but gives a lower sync/attainable upstream. Clearly I don't know what sort of errors it is giving. Reported SNR seems higher with the Zyxel, the HH5 seems to be constantly at just below 6dB which may account for the sync speed. I also seem to recall a setting on my Asus DSL-AC68U which allowed you to force a higher downstream rate at the expense of upstream, and I wonder if this is what the HH5 is doing?
Using the latest AAKL.7 firmware, which allows me to run both DSL Stats and HG612 Modem Stats, so I am assuming the single Telnet session limitation has been removed. Also with the Billion I had to revert to the 0.38 DSL driver firmware to stop excessive errors which finally dropped my fastpath connection, but the 0.39i driver in the Zyxel seems to be performing flawlessly.