Check our presentation here: Findings and solution for the Snapmaker J1 clogging problem - #108 by Mechanikus
I had measured the temperature drop inside the hotend there since I wanted to know if replacing the fan with the more powerful one from the E3D V6 would help. Cooling works perfectly fine though - the only obstacle I could find was that the critical area just above the thin area of the heat break is cooled slightly worse if you fail to follow the instructions and do not insert the hotend until the bottom of the heat sink is flush with the lower end of the housing.