Thermostatic heads and bodies are subject to strict standards (EN215). The temperature offset is calculated; this determines their efficiency. It is the value that is entered into calculations of energy consumption (used in the French RT 2012 regulations).
The figures for COMAP heads are available from the Certita certification organisation : http://www.certita.fr/marque-certita/variation-temporelle-des-robinets-thermostatiques