Be aware that the new starters will be an integral part of the flywheel — no starter motor with a little gear that engages the gear teeth on the outside of the flywheel.
So starts will be silent.
Once the engine is running the starter becomes the alternator.
The electrical system will be 48 volts, which allows more power for a pound of machinery including wiring.
I think mileage improvement will be simple: go to diesel and make the engines less powerful. No rocket science. The vehicles might even become cheaper to make.
Will consumers accept less power? Depends on cost of fuel and cost of cars that can deliver the required mileage and still have the power.
Even including the heavier 24 cell battery vs 6 for 12v?