Your alt only needs to cover your average current draw. Also your stock alt can charge as manny batteries as you want to hook to it as long as it can cover the average current draw os the car+system, it won't have any problems.
There has to be a voltage drop below a certain threshold (the drop is what causes the dimming) before the regulator ups the output of the alt. With a good sized bank of batteries, the batteries absorb the shock load of the transient current demand and minimize the voltage drop. Then the alt merely need replace the charge from the battery. With very transient music, the transients are shorter than the breaks between them and as such the average demand from the alt isn't really all that high. The system might draw 200A for a fraction of a second (guess what, your 200A alt won't do jack during this period) but the alt has a second or two to replace the amount discharged during that brief spike. It doesn't matter if the amount discharged is taken from one battery or 10, the amount is still the same and that same amount needs to be replaced.