Comcast HD Bandwidth

The bandwidth varies form channel to channel, if you have a SA8300 you can access the diagnostic page while viewing and it will show you the rate of the different channels. After having D* for over 8 years I can tell you that Comcast(in my area)blows D* or E* away,the difference is night and day
 
Well, I wouldn't say they pass exactly what they receive. The OTA signals use 8VSB. Comcast uses 256 QAM. They have to convert the 8VSB signal to 256QAM

No, that is not correct. In your, and mine, area Comcast gets their signal directly from the stations at Sutro before the transmitter. They have the same raw signal that the OTA stations get and do their conversion to 256QAM and that signal is not 8VSB.