How are you testing? If you're using Speedtest.xfinity.com...it defaults (for some really, insanely STUPID reason) to IPv6...rather than IPv4. Mine was showing up as .78 Mbps download... I, like you all, am only getting around 200 Mbps on the 400 Tier. The tech was just here and also suggested a DOCSIS 3.1 modem. I'll spend the extra $$ (I just upgraded my DOCSIS 3.0 modem, which should be more than capable, as it, along with my router, are capable of 1.4 Ghz speeds), yet i'm getting SLOWER speeds than before the modem or tier upgrade. Something ain't right...and for the pat answer to be 'It must be YOUR equipment...OUR stuff is great!' that's lame. If I spend double the price for a 3.1 modem...it had best work...FIRST time.
Hi Batman_66, thank you for working with me via private message and glad after resetting your equipment you are now getting the proper speeds. Please feel free to reach out if you have any questions or concerns in the future.