Fantastic! 415 solved the slow arcs. 
Along the way I noticed that the steppers are set back to A4988 in 415 with steps/mm to match. I set it back to DRV8825 with 200 steps per mm.
I also did M502 and M500 to reset any other settings just in case.
OctoPrint gives a warning about temperature protection when extruders is 0. I鈥檒l probably submit a PR to upstream Marlin about this. (Done: Fix where Extruders == 0 is considered thermally unsafe by vector76 路 Pull Request #16459 路 MarlinFirmware/Marlin 路 GitHub)