Thanks for all the information, looks like a decent starting point.
I think it would be good to see the result of a speedtest with SQM disabled (as baseline) as well as one with your current SQM config. Maybe dslreport's speedtest? then see these instructions/recommendations...
Overall, I guess that you could/should try ack-filtering (due to docsis' bursty MAC) and potentially per internal IP fairness (to isolate your gaming rig from other users), but let's start with the speedtests.