I wrote this up on my blog, where I can intersperse text and graphs a bit better:
http://burntchrome.blogspot.com/2015/06/htb-rate-limiting-not-quite-lining-up.html
Basically, I ran a series of tcp_download tests, using increasing ingress rates with sqm_scripts, and then used flent's box-plots to put the results into a combined image for comparing.
On the 3800, it never meets the rate, but it's only off by maybe 5%. But on my new WRT1900AC, it's wildly off, even over the same performance range (I tested it from 80-220Mbps rates in 20Mbps jumps, and saw from 40-150Mbps.
I have no idea where to start looking for the cause. But for now, I'm just setting my ingress rate MUCH higher than I should, because it's working out to the right value as a result.
-Aaron