COX won't update customer provided modems after screwing up Gigablast speeds....

I can confirm that I am having this same issue, also on an MB8600.  Speeds are topping out between 300-350 Mbps whereas before I would be able to get to a little bit above 900 Mbps.  However, I am still on the 8600-6.1.1.19 software version (though based on your previous comment, it doesn't seem to make a difference). 😞

As a side note, I noticed that even when things were fine, enabling IPv6 in my router would degrade top speeds (down to the 300s, I think it was?), whereas going with strictly IPv4 would result in normal tests ranging between 700-900.