The Teslas will suffer longer battery charges (much more than 20% longer) and significantly reduced battery life if on this new firmware. It would take improvements in hardware (or drastically reduced durability) to make it available for everything.

I just wish I could pick these modes for my phone, especially if it's a non-repairable battery. My Lenovo laptop actually has something like this - it puts the laptop to sleep at 20% and only charges to 80%, unless I choose otherwise.