This is my OPi Zero before latest updates: http://sprunge.us/Uacg (you can see at the end at which versions the installation was regarding u-boot and kernel).   We had a known miscalibration of thermal data on H3/H2+ for almost a year (IIRC it was related to reading out the so called SID from the CPU and since this was broken thermal readouts were 10°C-15°C off -- too low). I now did the upgrade but temperatures didn't change that much after the reboot with most recent u-boot version (