I have not done an exhaustive tracking of what was changed over time (but I suspect JDW has, and will chime in shortly). I do know that the flyback was upgraded fairly early on, so incidents of HV failure dropped quite a bit. However, the yoke connector problem (triggered by heat) was pretty much a constant over the life of the Plus. The basic switching supply topology did not change (AFAIK), and there were no component upgrades that made much of a difference in efficiency. So, the amount of heat produced likely remained constant. Improvements in longevity mainly came from component upgrades aimed at reducing their sensitivity to the heat.
If JDW (or anyone else) has specific knowledge of power supply changes during the life of the Plus, I'd love to hear from them.