Good post. All this info is out there, going back to the E9X days, but this makes it easier since it's all in one place, especially now that the F8X will only be offered with the HK system.
I'll make a few comments:
I believe MoBridge makes a unit now that does DSP; you get very nice parametric EQ, time alignment, level control, etc, so that eliminates the need of a piece of gear. MoBridge-->amps.
If recoding to HiFi, you have accept the risk that it can, and likely will, be coded back if the dealer needs to do something like a large software update.
While I agree that running cables from the head unit to the back is relatively simple, to do it cleanly would require removing a lot of trim, and is much more labor intensive than the BavSound mid/tweet upgrade.
I think the way Dave did his system is a great compromise. And make no mistake, his low end is just about as good as it gets for our cars, without making significant modifications (some of which are not reversible), or a lot of custom work. The JL 10W6v3 is truly an SQL sub, meaning it reproduces faily accurate, tight bass and has good output. Noticeably better than other options out there for our cars.
Logic7 in these cars is not just highly overrated, it's pure garbage. A decent stereo reproduction is worlds better, even for movies.
The BimmerTech amp is great.
It's really not that hard to upgrade the HK system, just costly. But completely worth it. Even at 5k, you'll get more bang for your buck than a lot of other upgrades, IMHO.
The cubby under the trunk floor seems like a gift from BMW for people wanting to upgrade the audio. It's a great place to put equipment, and it's right next to the battery
