On a R7800 (dual core @ 1.70 GHz), I did not observe a difference between 18.x and 19.x-rc2. For me, downstream is CPU-limited to about 150 Mbps using cake/higher with another setting (see the thread I linked at the end of this post.)
Are you certain the change is related to the version? Note that bufferbloat tests should be run a few times to get an average value. See this thread.