I just finished a trip in early March across Baffin Island in the high Canadian Arctic, our daytime/nighttime temps were ~
-35/-40C and we camped the entire time, so the gear was subject to cold the entire time.. I was using a 5D3 and 1Dx (loaner from CPS) Not sure if it had had the modification described, but I experienced no problems with either. My discussions with Canon prior to the trip about the cold and also the two C100 video cameras was the temp rating listed in specs is mostly the battery specs.. I kept batteries for both camera in a vest in between layers, and thus was able to swap warm batteries when needed, (about once a day) The C100's experienced issues with the LCD screen going black from the cold.. We did not try it but I think hand warmers, judiciously placed/taped would have solved that problem.