FWIW, DTD, that isn't necessarily about just the modem "being fast." Comcast clearly has not provisioned it properly to give you the speed you pay for. Which is awesome for you, but is also VVVVVVVVVVVVYMMV.
Actually, here's how it works: Comcast guarantees x mbps for a given tier over their
equipment (ie: their modems) and network, which typically operate over 8 QAM channels downstream and 4 QAM channels upstream. They are DOCSIS 3.0 compliant and can supply additional QAMs only accessible by third-party DOCSIS modems, which are capable of providing additional bandwidth but ultimately get throttled by Comcast's own equipment upstream.
I pay for the top tier which traditionally varies from 85 mbps to 150 mbps depending on line conditions and distance from headend. In my case, they are only able to advertise it as 105 mbps. Using the additional QAMs from a 16x4 modem, they are able to provision me the full 180mbps, the maximum they are willing to provision per coax line in my area. It cannot be obtained using their traditional 8x4 equipment in my area. Additional QAMs in lesser speed areas can often still provide a performance boost because they can broadcast over additional QAMs. The speed boost part is YMMV, but it never hurts!
What DOES hurt is an overheating modem, which can throttle itself to keep temps down (in addition to costing you a few extra pennies a year). This is why I don't recommend the popular SB6141. Find another brand that will keep cool under load, or at least consider keeping it somewhere with good airflow. I was particularly drawn Netgear's lineup because of their cost, dim LEDs, and manageable heat.
I work for a company recently acquired by Comcast.