These cars do put significant pressure on the battery system to perform, not the least of which is the Start/Stop function, I never used it much (almost never) in my 981, still you make a point about repeated engine restarts after depletion while stopped at traffic lights...
If I understand it correctly, auto start/stop feature does not really stress the battery. The high starting load is very brief (<1 sec) and if you look at the energy used per start it is relatively minimal and is quickly replaced by the charging system. Also, it is an easy start since the engine was just running, up to temp, and well lubed. The battery is never even close to being depleted by a start.
From my experience with BMW X3s, going back to 2011, the electrical systems are very robust and with 3KW alternators and big ass AGP batteries. The electronic voltage control allows the charger to charge and keep up with high loads even at idle (that is precisely maintained with the engine computer even at high alternator load). I had my my 2011 X3 in upstate NY in a town on Lake Ontario. The winters were very harsh. When I left for work in the morning I would first start the car and then immediately turn the heater fan on high, the rear window defogger, the headlights driver's seat heater, and steering wheel heater. Then I would wipe the snow off the car. I noticed the voltage, even at idle with this large load, remained well over 14 volts. This meant at idle the alternator was not only keeping up with the electrical load, it was charging the battery. My Boxster appears to have an equally robust electrical system (sized appropriately for the car) although I have never had to test it in harsh weather.