Power Vs KWh

Regarding kW (power) versus kWh (energy) of MS, I think there would be some correlation between health of battery versus the motor kW output. This is gleaned from the P85D performance being battery SoC dependent. Non-performance MS is less limited by battery SoC for motor kW output, therefore I would not worry too much. I also hope you don't lose 30% battery capacity after 6 years and 120,000 miles.

I am pretty sure that your battery will still be a 400V battery or very close to it at full charge and there will just be less total energy so you will still do 4.2s anywhere above 60% charge or wherever it drops off a little bit now

Apparently the battery is still 400 V, but the voltage sag under high load is greater as a result of internal resistance increases in the cells. This equals less peak power. My car has seen 3 85 kWh packs over its lifetime. I can tell you that the peak power output varied pack to pack.