Apples and oranges....
1. power electronics are liquid cooled in the DUs of S and X (3 for sure too)
2. Power transistors (igbt) did develop in the last decade a lot (less thermal losses, less saturation voltage e.g.) and they are "higher integrated" in the meaning "one part - far more output power), also the driver boards responsible for the "driving" aka switching of the igbts have become better...
3. one can guess about the the software controlling the driver circuits for the power gen. probably far more sophisticated
4. Model 3 will use a different kind of motor! Still a 3 phase AC but with a permanent static rotor which gives some advantages concerning eddy current (hope I translated that well) but also needs more electronics in the power department...
roughly I would say the PEM is like from the age back where tubes were used and the S/X are up 2 date stuff