to be fair i did get upwards of 1gig when i had the service. this was with both a cm1000 and a sb8200 (because they said my cm1000 was the problem). unfortunately, the modem stops providing a valid IP every week or two and after months of that bs and calls/hours wasted with comcast i rolled back to 400mbps service and the problem went away. i work from home most of the week and can't have 6 hour outages every week or two.
is your host (laptop, workstation, whatever) connected straight into your modem or to a firewall then into your hosts?