Yes, exactly that one. I've ordered the Artic Alumina today.
Seems like it's worth the effort then. But my question is: if a heatsink falls off, is it normal for the hashboard to stop working?
Yes due to overheating.
Here's temp range from Bitmain below for s9
Range: 65 - 115
Chip max. 135
PCB max. 90
Source: https://support.bitmain.com/hc/en-us/articles/360005088914-Miner-normal-operating-temperature-rangeIf the chip temp is more than 135 the hashboard will stop hashing due to overheating protection.