The article in that link has obviously been around for a long time but it is really a half truth. It takes the current drain to an extreme and depicts it as continuous. It also is using a tiny motor that gets tired to turn things as opposed to a car engine. The cyclist cannot keep up to begin with, that is not because the alternator was tapped out it was because the engine was. Once the battery went into discharge the guy is fighting a losing battle. A battery that is partially discharged is a load until it is recharged. If the alt never recharges it, it becomes more of a load as it discharges further. If this "experiment" had some more horsepower on tap to turn the alt, the result would 1) be more representative of reality and 2) be completely different in that the battery would never go into discharge with that paltry amount of discharge.
This isn't an accurate representation of what happens with your car unless your average draw (music isn't hardly continuous and doesn't hardly present a constant high demand on the electrical system unless you only play "music" with long droning bass notes) accounting for the peaks and valleys in demand exceed the output capability of the alt. In reality the battery discharges very slightly with a heavy transient, mostly because the alt cannot respond fast enough, and recharges almost as quickly in the time between transients. An extra battery works in the same way as the pressure tank on an air compressor. Try running an air tool with a high peak but low average demand for air, like a nail gun, on a compressor without a tank. It won't work worth a **** and to a point, the bigger the tank the better it will perform. Your car's electrical system is the same way. To a point the bigger a current reservoir the system has to draw from the better it will handle peaks. The problem comes when you get the reservoir too big and the alternator gets well behind in keeping the reserve filled. Adding a single battery isn't going to hurt anything and will help stabilize the voltage for a system that needs to deal with transient demand spikes but does not have a high average demand. If you are getting to a point where the average current demand is exceeding the capability of the alt to keep the battery recharged, its time to upgrade the alt.
600ish watts is a very modest system and the average current draw will be within the capacity of the alt to handle under normal conditions. Where you might run into problems is with the headlights on, the AC on and the music cranked. If it is really a concern, I would find a Class D amp to run the subs to minimize the current draw there and move your current sub amp to the front speakers, continuing to run the rears off the deck.
As far as David Navone and Richard Clark, they have also published articles that try to claim that grounding to the battery is bad by using what amount to a current detector to portray the battery post as a noisy ground when in reality it only showed that a lot of current flowed there. Their articles are perfect examples of a little information being a dangerous thing. I don't know if they actually believe what they say or if they are just trying to make themselves look more knowledeable or what, but they have propagated some completely false information over the years hidden behind a thin veil of fact and actual science.