I'm just like everyone else who's encountered this problem...
I upgraded to Gigabit speed because I figure with all the devices we have at home, more speed the better. So, I purchased an Arris SB8200 as it was the only DOCSIS 3.1 modem I found. I'm using it with our Netgear Orbi Wireless Mesh System.
I have a computer plugged in directly to the modem and I'm only getting on average 350Mbps/40Mbps, while my wireless I'm getting 250Mbps/36Mbps.
I've tried to talk to a Comcast Agent to assist me, but unfortunately they cannot figure out why my connection is slow. Before I take a day off and have them send a technician out, I'd like to see if anyone here can attempt to assist me.
Here is my modem information:
I just don't want to end up paying for something that I can't even get...
I am fighting this battle too. The rental modem will get 940 Mbit but the SB8200 only gets to around 600. So far Comcast has not been able to help. Also, initially I was limited to 350 Mbit until I turned off traffic metering in my NetGear router.