Are SSD Heatsinks worth it or a waste?

Yes and no.

Unless you do large amounts of I/O, you will likely never overheat your drive controller and therefore won’t see it throttling. As mentioned above only the controller benefits from cooling while active, the flash chips want to be warm while in use and cool before power down. See here for more info The Truth About SSD Data Retention.

It could be argued that spreading the heat from the controller to the flash chips is beneficial (warming the flash while cooling the controller) I would just ignore thermals completely.

Despite above advice I have used a few old ram chip thermal pads (1.5mm thick) on top of my SSD and it sinks heat into the top cover below the “B” key. This cools the controller only and I never throttle even in synthetic tests.

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