I have used the Kinetik batterys in several installs, including the installs in my M3 and my 540. They are good batterys, and I can tell you they actually make the systems sound better. That being said, I dont think that I would add a second battery, just replace the current battery with the correct Kinetic one.

As far as wiring goes, I have built many systems in all models of BMW's and on only extreme occasions have I not used the actual BMW speaker wire. I dont ever think I have seen (or heard) where it actually makes that big of a difference.

Do you have a system planned? I have never upgraded an alt in any of my cars with the exception of my VW GTI when I was going for an dB machine. I am currently in the process of assembling product for my X's upgrade, the Kinetik battery will be included, however I will not be doing anything with the alternator.