I would love 4K raw from the R5, but wouldn't it mean it'd be cropped? Not sure if the R5 can output a oversampled or pixel binned AND Raw image. This is why it already isn't available in camera. Unless the Atomos can do some of the processing? I don't know how that would work.
I doubt overheating will be an issue in a practical sense. There was a time when having the LCD screen on would cause overheating on the R5 even when outputting to a Ninja, but that's been fixed in a firmware update. 4K120 is the biggest culprit to overheating, so it might be technically possible to overheat with an external recorder, but I don't even know if this feature will be available. Currently the announcement says 4K120 is available with a couple Z-Cam models, and Sony cinema cameras utilizing the SDI Kit.
Jeah, I think its not likely that the camera can put out 4k RAW. But it appears possible to me, that the Atomos could transcode the 8k RAW to 4k RAW. Its not "real" RAW anyway. Prores RAW is already a processed image thats allready debayered if I am not mistaken. So another processing in the Atomos doesnt sound impossible for me
Right now 4k HQ overheats if you work for some hours with on and off recording, even if you only record externaly. The 4k HQ is noteable better than the 4k normal mode - so it may be worth the risk in a controlled enviroment if the client ist breathing down your neck.
I am not sure about the amount of processing going on in the camera - but I think putting the image online out over hdmi without processesing it from 8k to 4k would take less processing power and less heat. So maybe 8k RAW external would be less prone to overheat than 4k HQ externaly

If so, the Atomos would be a very welcomed upgrade and I would sell my current atomos V =)