when my car is on and my alternator charges the battery to whatever volts, do the max amp or watts have an effect on performance. or do these numbers only apply to when you play the stereo with the engine off?

im thinking of getting more audio equipment and want to keep light dimming to a bare minimum. right now on jsut a 1200 watt amp for my sub and the stock JBL amp i only get .1-.2v voltage drops. i dont plan to upgrade the stock 80amp alternator anytime soon but am thinking of getting an XS or Kinetik under the hood since my interstate keeps losing its charge after about every 6 months give or take.

also i have the big three done

They play the same role with the engine running or not. Bigger batteries will give you more reserve.

Time to upgrade batteries and alt. if you are going to add more amp load.