Comcast's business practices aren't great, but I will give them credit. I get every Mbps that they claim I am supposed to get. This is plugged into my router (Turris Omnia) and my cable modem (self-owned Arris SB6190).
http://www.dslreports.com/forum/r28685577-Speed-comcast-spee...
Just Google "puma 6 issue" and see the vast issues that chipset has, and checkout http://www.dslreports.com/tools/puma6