"And because all OEM speakers are 2ohms and 99.9% of all aftermarket component sets in 4" are 4ohms (only the BSW sets are 2ohms) the effective usable volume will be cut in half"
Here is a photo of a BMW OEM speaker from an X6 with premium audio built this year. You will note that the label states quite clearly that it is 4 ohms. The same applies to all the other speakers in this car apart from the bass units under the seats which are 8 ohm. Individual audio speakers on the X6 (which I used as upgrades) have exactly the same impedances as premium, not a 2 ohm in sight.
I haven't yet dismantled my wife's F25 with premium audio but I would be amazed if it had any 2 ohm speakers. And to correct someone else's point, premium audio on the F25 is not Logic 7, it has a Lear-built amplifier that is similar to the E70/E71 premium audio.