Depends what you are expecting to achieve as to whether this is a worth while expenditure of your time and money. If you have a good home sound system (i.e.
not Sonos or Bose, but rather a Hi-Fi seperates system comprising of good brands such as Elac, Linn, B&W, Tannoy etc) - then you are really going to struggle to ever achieve the kind of audio quality you can achieve through these systems. You can match them on outright volume, and you can get a lot of bass (but low in quality). But you are not going to achieve the clear stereo soundfield, the clarity, the timbre and imaging.
The car is an inherently terrible place to try to establish good sound quality. Practically everything about a car is working against you. The doors where the speaker drivers are mounted are flexible, even in a good quality car like an audi. Push on the door panel and it will move. The same happens when you push a speaker hard, much of the energy will go into flexing the door panel rather than moving the speaker cone. This is very undesirable. There is a reason expensive speakers have large, heavy, solid enclosures. The doors are also obviously not designed or braced properly to be a speaker, there are all sorts of bad, crazy vibrations, standing waves and pressure variances inside the door when speakers are running. In the best in car audio systems such as in Rolls Royce's, Bentleys, or even the Audi A8 , they try and alleviate this to some extent by having a dedicated speaker driver enclosure for every speaker driver in the car(which is partly why the B&O in that car costs £5k), this helps a lot as opposed to freely suspending the speaker drivers in the door panel as per 99% of car audio systems including the B9's B&O system.
This is not to mention the lack of clean power supply from a car, the vibrations from poorly fitting door panels, the lack of anywhere good to place the speakers in a proper stereo configuration, the resonance from the car's body, the vibration from the engine and road, the interference from all the other electronics in the car.. the list goes on and on. You can have a good sounding system in a car by spending a lot of money, but you can probably match the bespoke audio system in a Rolls Royce for 1/10th the price with a home system, and it will still be better than the Rolls Royce system in some ways. So if you have an appreciation for all of the above, then the cheapest and simplest options is to not even try, because achieving anything great will be extremely expensive and difficult.
If all you want is something "pretty decent" that will go loud and not distort, then you can reasonably expect to achieve that by just upgrading the car amplifier, throw in some decent after market speakers and add a bunch more anti vibration and sound insulating material to the door panels.
I say all that having ordered the B&O myself. There is no way I am going to attempt a custom audio job myself, and the B&O should atleast provide passable audio quality. Some here do not seem content with it, but I suspect atleast one of the following is true: They are choosing a bad audio source 2) They are using low bit rate files 3) They have not allowed 50+ hours for the system to bed in 4) They do not know what true Hi Fi actually sounds like, and prefer smiley face EQ'd consumer speakers like Bose where there is a whole lot of bass and not a lot else.
Or maybe the B&O system really does suck, I will found out soon enough