Far more important than "lots of surface area", is "lots of useful surface area". The SLK-800's fins are too high and too thin. About 90% of the fin height is totally wasted and not doing anything. Sadly 90% of the flow correspondingly goes to those regions, meaning that only 10% of the flow is actually doing any real cooling.
Chop the fins down to say 3-4mm high, and the fin area to say 25mm wide at best, and you'd come close to having a somewhat decent performing waterblock out of it.
Heatsinks are designed to specifically deal with air's poor ability to soak up heat. They offer lots of finnage over a huge area, because air needs that much area to get the job done of taking the heat away.
Water is stupendously more efficient at soaking up heat than air, and correspondingly, you can do come pretty amazing things using a surprisingly small amount of surface area in comparison to air-based heatsinks.
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