Another Arris SB8200 slow download performance problem

I recently upgraded to the Gigabit tier and purchased an Arris SB8200 to take advantage of it. As with a number of other posters on this board, the device syncs up with 32 channels of DOCSIS 3.1 download (blue light) and 4 channels of DOCSIS 3.0 upload (green light). The upload performance relative to my old 250 Mbps service was a big improvement - went from ca. 6-7 Mbps to around 40. But the download performance is exactly where the old service was, about 330 Mbps in Xfinity speedtest.

While there have been a bunch of other posters with similar problems, AFAICT nobody actually has provided a solution to the issue - some of them seemed to just live with it, others to replace the modem with a different model. I've talked to Xfinity service multiple times (after the obligatory reboot-the-modem-multiple-times hoop we're forced to jump through), but while they're all perfectly nice, they're also all perfectly useless - I keep asking if they can confirm that the service is not being capped at their end, and they say there's no sign of that (possibly the most annoying part of this is that they keep sending me letters telling me my modem isn't taking full advantage of my service - gee, thanks, guys).

So coming here in the hope of some useful advice. For reference, the modem is normally talking to a gigabit router (ASUS TM-AC1700) and that to my home network of Linux machines, all hardwired with Cat6. I routinely achieve internal transfer speeds around 850 MBps so that's not a limit. I've experimented with directly coupling one machine to the modem with no noticeable improvement.

Dump of the modem connection state follows. Any advice appreciated.