But per Carnot, maintaining a fixed temperature differential against a fixed thermal resistance is cheaper (requires less work) at a higher temperature (1/(1-(Tcold/Thot)) is the carnot-limited "coefficient of performance" for a heat pump). So theoretically they should be able to maintain the design differential even at higher outdoor temperatures, causing indoor temperatures to raise from ~21°C to 26°C comparing the "still works at 35°C" and "40°C happens regular in $current_year".