CM1000 only getting 400-500Mbps with Gigablast

I've got a CM1000 and Gigablast but I only ever get 400-500mbps. The connection is stable, it's just not hitting advertised speeds. I've tried changing the coax cable, moving the modem to another room in the house, connecting directly to the modem, etc. The speeds are always consistently about half of what I should be getting.

I contacted support a few times. The last two people I talked to both said the signal looked good on their end. One said they were seeing 11000mbs to the modem, and no packet loss, so the problem must be with the modem. The other similarly said that the signal looked good and there was no packet loss. One said they were getting red flags on the modem, specifically HomeRF, OFDM Status and Channel State. They suggested it's a modem configuration issue but, since it's a third party modem, they couldn't do anything.

Thing is, Netgear doesn't have much in the way of support for this kind of thing so I've been looking for information online and I'm seeing a lot of similar complaints with stats similar to what I'm seeing on my modem that are always attributed to line problems. For example, the fact that I'm only bonded to the first 20 channels is often attributed to a filter somewhere on the line (there are no filters or splitters inside the house, as far as I am aware). Some of the error messages I've been seeing in the logs are also frequently attributed to line problems. No offense to the reps I spoke to, but I suspect they're more customer service than actual technical experts. I was hoping for a second opinion and maybe some more information.

Here's what my modem is showing: