[tonyb]

That's not how QoS works on a DOCSIS network. DOCSIS QOS is controlled via the DOCSIS config file and is applied to any cable modem, it is part of the standard that all cable modems - so any QOS that would be applied to Comcast supplied modem would also be applied to a customer provided modem.

I don't have any specific knowledge of how Comcast does or doesn't do throttling on their network but it is very unlikely that is happening via the CPE. They may be doing some QOS via PCMM (PacketCable MultiMedia) but that is often used to manage congestion on the cable network and not the backhaul/transport side.

[tristor]

You're correct that there is also DOCSIS QOS being performed and this happens regardless of who supplied the modem, this affects traffic between the modem and the head-end, but past the head-end traffic is shaped using other techniques. QoS is protocol specific, and based on the behavior I've observed I believe they are doing application classification and doing IP QoS on the CPE in addition to any DOCSIS QoS applied regardless. I'm a little rusty on my understanding of the DOCSIS specs, but this is a L2 protocol and QoS applied here isn't based on application classification, AFAIK.