Modem Buffers (Score 1)

You are right on #2. However, the largest contribution to ping times is not the Tx buffer at the ISP, but the Tx buffer on your modem! As you all know, most broadband connections have much less uplink than downlink data rates. Most of the bb connections have like 128 kbps of uplink. If you try to send data faster than that, it is buffered inside the modem. It doesn't matter how you prioritize things at that point, because it all gets backed up in the modem. That is why QoS routers have some form of uplink rate matching. That means that you prioritize packets, and only send them at the rate the modem can forward them on. If you choose just the right uplink rate at the router, then you are prioritizing right before the packet gets sent to the ISP. Only if you do all of this right will the ISP's buffering play a major role. Of course, this situation becomes a little more complicated if you have a shared pipe leading to the cable head-end unit.