2. The costs of the transmission system operator shall be subject to an efficiency comparison between transmission system operators. ACER shall carry out that efficiency comparison. By 5 August 2027 and every four years thereafter, ACER shall publish a study comparing the efficiency of transmission system operators’ costs, subject to the protection of data considered by ACER to be commercially sensitive. The relevant regulatory authorities and the transmission system operators shall provide ACER with all the data necessary for that comparison. When periodically setting the allowed or target revenue of transmission system operators, the relevant regulatory authorities shall take into account such comparison and national circumstances.