Heat-sink as add-on?

[Andrew2]

Here is a enclosure for Raspberry PI made from aluminium with "cube" which connects enclosure body and surface of CPU. That might be the best passive heatsing from all the options

This looks nice and is quite expensive so once you bought it you already feel that it works perfectly (that's how psychology works). But if you start to measure then things look different. I've provided a script that sets up a whole monitoring solution within minutes that can be also used to monitor the performance effects of throttling and results are simple (and the same as with any other chip on this planet)

If you really want to do number crunching you need some ventilation. And most importantly a somewhat working heatsink on the SoC (DRAM and PMU doesn't matter in this regard). The most expensive heatsink won't outperform the combination of a cheap one combined with some airflow. One large silent fan mounted below a bunch of Pine64 with controlled airflow is pretty enough to keep A64 below 80°C so no throttling happens even with heavy workloads (Hadoop is none)