For anyone else looking, there's quick-and-dirty notes on how to configure this back in the 1.7 release notes. (Search for "smart queue") https://community.ubnt.com/t5/EdgeMAX-Updates-Blog/EdgeMAX-E...

Note the comment "the actual rate limits will be set to 95% of the specified value". That explains why I saw a 5-10% dropoff in throughput when I enabled it with honest numbers.