Back of the napkin speculation here; the 645LT can cool the 3950X and the 9900K barely at stock (80-90+C easily). Those two parts are around 130-145 watt parts off the entire CPU package when all cores are fully loaded. A GPU on the other hand has way more power with the TDP of a 2080 super being 250 Watts, but I'm uncertain if that's overall or on the die. My guess is the 250 watts accounts for everything from the PCIE to the two additional 8 pin connectors. This means some of the watts will be dumped as heat from various components from VRM to GDDR6, and not just being released from the die itself.
So you might be able to get away with a 645LT. There are many AIO 2080s that come attached with a 120 radiator along with a blower style fan, and many reviews report that the fans are barely audible while the GPU is cool and quiet, meaning they aren't going full tilt and that the die is possibly using around less than 150-160 watts worth of heat (my reference here is that I had a 120 rad cooling an overclocked 2700 which had 155 watts on the package and the fan had to go full tilt). A 645LT will probably still require a stronger fan to compensate for less radiator - like the bGears B-Blaster 90mm fan that can go north of 3000 RPM.