These are just some raw results, I haven't done any tuning (need to do other things today), but I finally got around to contacting Comcast support, and getting the modem switched to full bridge mode. Since it's their combo modem/AP/router/voip unit, support has to do it on the backend. But live-chat support was able to do it (and able stay operational through all the ip address changes, which was nice).
But the WNDR3800 is certainly at (past) the edge of what it can handle at these data-rates when using the simple.qos scripts at:
download (ingress): 100000
upload (egress): 12000
squash ingress DSCP
ECN enabled in both directions (but not enabled on the local end of the test)
So, here's a bunch of graphs: