a lower cca battery will leave you stranded if the car isn't driven daily or weather is too cold. it's not worth it.
it's a shame that you have to get dealer to change battery. a battery is about $100-150 and bmw is screwing you for making you go to their dealers.
there are so many things on bmws that don't make sense - RF tires that wear out 2x faster, etc. I come from Porsche, Lambo, Ferrari and MB. I think BMW should focus on engineering for better reliability and safety.