I think the main issue that you may be missing is that Porsche says it should not be over 14.5 volts for any or any extended period of time. If they're charging the car above 14.5, it's because their electronics know what their batteries can absorb. When you plug your ctek into it, if you're in polar mode it's going to apply 14.7 volts for some period of time that they have determined is correct for their usage. This usage is for Power Batteries not for auto batteries.I’m not saying you’re wrong about anything you’ve written here. What I don’t understand about this entire discussion is this: Your alternator under certain conditions bumps the voltage up to 14.9 or 15v. I’ve seen this on my CGTS. The electronics in the car are all 12V. The only reason to bump up the voltage (that I am aware of) is to create a faster charge on the AGM battery.
Whether this voltage is coming from the alternator or a charger makes no difference. It ends up on the negative terminal at the battery either way (because that is the whole point). There may very well be a second or third negative lead from the battery which limits voltage or current for whatever purpose on other circuits, but that shouldn’t matter one bit to how the battery is charged.
I’m probably missing something.
This is all a bit of guesswork because I think there's a lot of unknowns in this area. That's why I am deferring to use the lower voltage. It won't hurt as its long-term storage. If I run it at the high voltage there is potential for damage based on what I have read. That's it.
MOO & FWIW