let's consult ohm's law for this.
and we'll address the above post first (not picking on you, you just offered more numbers to use).
let's say you did achieve 6kW of output power and the amp(s) is/are rated 6kW at 14.4VDC.
at 6kW of output power your amp(s) will require just over 7kW of input power (assuming minimum 85% efficiency which is unlikely at full output).
if you were able to maintain 14VDC at this power level, you'll draw 500A. since the alt can't do that (assuming you were not at idle but holding RPM above 2500 to get rated alt output) the voltage will drop to the battery voltage - 13VDC at best which will continue to drop to 12.5VDC or even lower as you sustain this output. as voltage drops so does power output.
to the OP: what you can run on a 230A alternator is determined by many factors. you'll only get that by holding the engine RPM above 2500 RPM. and when you ask for full output the alt will create extra drag since it gets power from the engine. keep in mind that the alt gets harder to turn when load increases.
the alt is capable of providing around 3kW total, full load at speed. but the car needs some of this. how much is actually available depends on the use of other power draws. lights, turn signals, wipers, windows, A/C, fans, etc. are all loads. on a nice, cool day you typically use less than hot, muggy, rainy night. the latter means you just keep the system at a lower volume.
so let's say you are looking for what system you can build that will maintain 14V while at speed, and on a nice day. and let's say your car only needs about 40A or so. that leaves 190A for your system which is around 2600W input power which correlates to around 2250W output power (class D). as you come to a stop the alt output drops to about half, which correlates to you being able to achieve around 1100W of output power while maintaining 14VDC.
that's not to say you can't get more power out of amps, just that the voltage will drop, so will power output, and you'll have to tax the system even further reducing system voltage and power output. it's a viscious cycle. with extra batteries you can maintain 13VDC or less, so you just keep this in mind when choosing and loading amplifiers. extra batteries will not help maintain 14V, but instead are loads which further tax the alternator.
on a day where the car is demanding 70A, at idle you are down to 500W of output.
