Exactamundo, but bizarrely with the R5 it doesn't work - in use it's a car crash. I bought it specifically for unlimited 4K HQ in crop mode but confusingly it seems it only plays with the EOS R. Wasn't just a one-off either, it's a known non-starter for the R5. Canonrumours speculated that Canon will release their own speedbooster
It works! Just tried it:
15 mins recording 8K IPB to CFExpress, overheating icon
Day +1, pulled battery
Inserted battery 30 secs later
No overheating icon
Turn off camera
Turn on camera, change date to current date, no overheating icon.