I don't think there is a need to add the 8C temperature offset in i8kfangui. It's ATItool that reports the wrong temperature. Here's a little trick to check the real temperatures:
Suspend your laptop and leave it for a while (overnight). When you resume the next morning, notice the temperature of the CPU and GPU. They must be the same.
If you add the 8C offset, the GPU is 8C hotter than the CPU, which can't be right, since you know they are at the same temp.