I've read that a somewhat typical specific heat capacity for EV battery packs is in the range of 800 - 850 J/K*Kg
Your number is about in that range
I think it is ballpark correct to say that after Supercharging, the car wants to reduce pack temperature about 15C quickly (say, within 20 minutes). If the heat can be utilized or stored elsewhere -- great. If not, it gets expelled to ambient. A 15 C drop in pack temperature using 825 J/K*Kg in an 80 kWh pack (Model Y LR) calculates out to 1.8 kWh, or about 5.4 kW over 20 minutes. After that heat dump, the pack has another 10C or so to 'spend' before the next Supercharger stop. Unless ambient is **really** code, I have read that a COP of ~ 2.0 is realized, which implies another 2.4 kWh of heat into the cabin.
All told, the Heat pump/Octavalve can transfer about 4.2 kWh of pack heat into the cabin between Supercharger stops without causing other performance issues. If ~ 40 kWh are sent to the battery every charging stop, this heat scavenging approach improves range about 10% so about 1/3 of the EV range winter penalty has been mitigated.
Amazing engineering, but as I posted above, the driver can help by giving the car a place to dump heat.