Success! The support team found that I also needed to enable Advanced->QoS->Application->DSL/Cable Optimization. Once I adjusted my WAN download and upload values properly (around 10% below Comcast provisioned values), I was then able to get a Bufferbloat grade of A with Speed test - how fast is your internet? | DSLReports, ISP Information.
FYI, and as is expected with Bufferbloat algorithms, the extra cpu overhead reduces my download throughput on the Surf Soho to around 100Mbps +/- 5Mbps as measured on dslreports (the Surf Soho is normally rated for 120Mbps).