If you know well your temperature range and power levels, you can build your calorimeter to make the delta T range that is most favourable for measurements, to any delta T per x Watts recovery, by adjusting and optimizing the volume of “coolant” and flow rate.
That’s true but remember if we remove too much heat from the system in order to measure the heat flow then we affect the reactor power. So there is an optimal point we have to find between these competing issues.