Arris SB8200 doesn't deliver more than 330 Mbps down on higher speed tiers

My problem is identical to the one previously reported here, which never seemed to reach a resolution:


https://forums.xfinity.com/t5/Your-Home-Network/Only-getting-20-30-of-1G-bandwidth/td-p/3280081

I have an Arris SB8200 CPE as the modem. For the last year I've been under the Gigabit tier, but never got more than 330 Mbps down as measured by speedtest / speedof.me. Attempts to diagnose this with support quickly ground to a halt as they could not run deep diagnostics on the modem and refused to do anything more unless I rented a Comcast modem.

Since I wasn't getting the performance I was paying for, I just downgraded to Extreme Pro (nominal 600 Mbps down). Measured performance down remains unchanged at 330 Mbps. The modem reports 32 locked QAM256 channels with SNR 38-41, which should AIUI easily be able to deliver 600 Mbps, but does not. It seems likely there is a misconfiguration or throttling issue happening somewhere between the cable coming out of my wall and Comcast's network, but with zero ability to diagnose that myself and support unwilling to do anything about it, I don't know that there are any options. But clearly this isn't a one-off problem given the link above (and other examples can be found without much trouble). Any chance of getting an escalation from the official Comcast employees in this forum? I understand the difficulties in debugging problems with CPE, but this is a Comcast-approved device and I am experiencing a problem that affects other people, so am hoping it gets more engagement this time.