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Supercharger protocol for diy CHAdeMO adapter - Page 8. It looks like the wire inside the car is #2 AWG. That dissipates 17 Watts per foot of wire now at the 333 Amp max current of a 120 kW charge rate. If the current scaled, that would mean 375 Amps for 135 kW charging and 417 Amps for 150 kW charging, pushing the resistive heating in the on-board cable to 19 and 22 Watts per foot. I am sure that Tesla did good thermal design and used high temperature insulation, but that #2 wire could be a limit.
I assume that the wiring from the Supercharger to the pedestal need to meet NEC. The car does not need to meet NEC, and the pedestal probably doesn't either. Just to carry 333 Amps per NEC, they need to go to 350 mcm wire between the Supercharger cabinet, and wire size goes up from there at higher currents. My guess is there is a limit to what even Tesla will do to save a few minutes of charge time.
The higher powers will certainly be useful to speed up charge time for the second car to arrive without needing to raise power and current to a single car. Of course, we can hope for that 110 kW-hr battery for the Model X that can accept even higher charge rates...