I also bought an Arris t25 last week and signed up for 1Gb access. It does not work. I then asked them to send me one of Xfinity modems, which I installed and I am using now and the same thing, the speed is close to 300Mbps.
I tried the suggestion of going to 600Mbps that is floating in the forums. I signed up for that and the speed is still 300Mbps.
The tech came by on Monday and checked to line to the edge of my house. He said all is well.
Spent hours on the phone a no one could fix the problem, even on their modem.
I am scheduled for some super-tech call on Friday, but I am losing hope.
I spoke to Arris, and showed them the QAM readout and they noticed that I was missing QAM 32. The display was always up to QAM 31.