I just upgraded my modem from a Motorola SB6121 to an Arris SB8200, anticipating that my area is going to be getting the new Gigabit speeds within the next few weeks, i went ahead and purchased a DOCSIS 3.1 modem.
I got the new modem activated, and I went ahead and tested it before I hooked it up to my network. I got an average speed of 30down and 5 up (yikes!), while my existing modem, hooked up to the network was able to reach our plan of 150 down and 20 up with no issues. I've tried speed testing with the new modem on two cable jacks in my house, with the same result.
So basically, with the new modem, i'm getting slower speeds than I was getting on my original SB6121.
Status and logs are attached below
The signal stats are fine. It appears that the new modem has defaulted to the Performance Speedtier because the correct config / bootfile is not being pushed to it.
Call 1-800-comcast and ask a rep to *roll and re-provision* the modem. Also ask them to confirm that the provisioning and account data in their database is correct and complete and that the correct configuration / bootfile for your subscribed to speed tier is being pushed to the modem.
Six month old dead thread now being closed.