Did you ever get a response from anyone or talk to anyone at Comcast about this? I just switched to the 1G plan and I am seeing thesame speeds with the same modem.
I'm having the same issue on a Gigabit plan. Last month I could get ~900Mb down and ~41Mb up. For weeks now I am getting ~400Mb down and ~31Mb up on SpeedTest and ~250Mb down and 29Mb up on xfinity speed test.
My Cable modem Power and SNR are nearly identical to what you posted as well.
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I finally called their "Executive Complaint Department" or whatever they call it. Got a seemling knowledegable guy. I said my signal is out of spec according to Arris - he said it is in spec according to Comcast! He had me disconnect all my equipment in my house and hook a PC directly to the cable modem - and it still didn't get anywhere near 1G! So, he said someone local will contact me on Monday and they will send someone out. Better than the last person I spoke with who said "If everything is ok on the Comcast side we will charge you $70"!
Ok, so tech comes out today, I tell him my story, he checks outside - replaces grounding block, but doesn't think this will really do anything, he tests cable inside at cablemodem - gets over 950M! He hooks cable back up to cablemodem, plugs his tester in to ethernet port on cablemodem, gets over 950M! I plug a desktop computer with a gigabit ethernet port - I get a little over 500M to both speedtest.net and speedtest.xfinity.com! He says "Well, the modem is fine it is your computer!" I can't really argue with him, so he leaves. I start trying to figure out what is going on. I use a router from Mikrotik that has a speedtest built-in, but it needs to go to another one if its own routers or a PC running the test software. I do some quick Googling and come up with a public server and BAM! I get over 900M from the router to the test server! So, what gives? The tech said he doesn't trust the Xfinity speedtest servers, but I am thinking none of them will work for anything over 500M - YMMV.
The problem with browser based speed tests is that there's often considerable overhead with browsers, or even background security software that could slow the speeds down. Some routers often run their own programs in the background that can stifle speeds.
To get a more reliable speed test, use the OOKLA Speed Test app, which you can download from https://www.speedtest.net . Test on an Ethernet connection directly to the modem or gateway.