Hmmmmm. I wonder if this is the reason for the heat transfer tape (heat sink) behind the bayonet mount on the RF lenses? It is not in EF lenses. See the teardown of the RF 50mm f/1.2L at lensrentals.The issue with IBIS and cooling is three-fold, and only the smallest part of the problem (heat generated by the IBIS actuators) is resolved by simply turning IBIS off.
The two main issues that aren't resolved are related to thermal mass, and thermal path.
Firstly, to optimise IBIS you need to minimise the mass of your sensor assembly, otherwise the IBIS actuators have to be much larger/more powerful to move the sensor as effectively. If you minimise the mass of the sensor, you're also minimising the thermal mass (i.e. the size of the heatsink), so your sensor will (for the same energy consumption) get hotter faster.
Secondly, because IBIS by definition means the sensor has to be able to move in relation to the lens mount, it also means the thermal path between the sensor and the outside of the body is seriously compromised (in the absence of ducted or peltier cooling). With a traditional fixed sensor, you can have a solid high density thermal path allowing heat from the sensor to be conducted to the body, from where it can be shed by radiant heat loss. With IBIS, you have at best a thin flexible connector with much lower thermal conductivity between the sensor and body shell, so it's much harder to move the heat out.
It may not be an issue with your style of shooting, but thermal management is a serious design constraint.