Sorry for my radio silence, been busy with some designing! Anyway @kenny made a new passive heatsink that fits to the upper-side of the SoC, and lo and behold, it works better! (of course you geniuses already knew that) 
I’ve only had the chance to really stress out the heatsink on an Edge-V at 30C, will attempt to make more measurements during midday when the temperature is higher.
Edge-V sees about a 10% decrease in throttling with the new heatsink. VIM3L experiences no throttling at 26C. The tester did report “momentary throttling”, but looking at the overall results, it looks like performance is stable.
You can compare the new results with the old results here https://www.khadas.com/post/passive-cooling-a-vim3l. I’m not sure why the latest results are choppier.