With a fully stock Ender 3, the limiting factor is almost surely the extruder, not the hotend, so I'm asking from a standpoint of having an upgraded extruder (mine is direct drive) but stock hotend. I'm particularly interested in PLA but other materials would be nice to know as well.
I seem to be able to get PLA rates up to at least 12 mm³/s without skipping, and possibly higher - I haven't conducted any rigorous tests yet but with models where the max speed hasn't been sustained I've had success with rates as high as 16 mm³/s. If nobody has answers I may try to make this rigorous and post a self-answer.
I'm asking this question both to choose speeds I can consider reliable and as a starting point for readers considering whether an upgraded hotend is worthwhile for increasing print speed.