My 8900AX-1600 arrived earlier and is now up-and-running. No scratches on it (!) and WAN speeds are roughly the same as the old HG612.
I guess it may take a little while to settle down but after 1 hour of operation max downstream and SNR are slightly better while upstream sync/SNR slightly worse, but not significantly.
But as I said earlier, the primary reason to change was for gigabit ports and better wifi. And to confirm - it definitely has 5GHz wi-fi.
ps I'm "kempshott" on MyDSLWebStats to try and spot any differences.
Thanks for that, I have been looking at your MyDSLWebStats as i am still interested in the 8900AX-2400. I am not 100% sure what I am looking at do forgive me if this is normal, looking at the 5 day trend it would appear that there has been an increase in FEC counts with the CRC counts remaining roughly the same.