A Kicker 400.1 and a Kenwood...? I doubt you will need a new alternator for a system of that size. Would it help? Sure, but probably NOT where I would spend the money first.

1/0 is way overkill for what you have now, but worth it if you planon upgradign into the future. Be carful of cheap wire. I have an example of "good" 8AWG wire that is the same size (copper wise) as a "cheap" 4AWG wire. Its NOT all the same...