one of the easiest upgrades to ensure that there is minimal loss in electrical current is to ensure that the grounds are upgraded.. hence why i keep plugging the fact into the car audio noobs. Factory engine grounds are not exactly the best - need to increase capcitance and fibre count in wire.
and yes i realise that 2kw has to come from somewhere.. and its from the alternator mainly.. not the battery but its affect is minimal overall as you say.
Increased battery size will ensure that the system has the capacity to not draw extensive power from the alternator.. handy when you have large audio systems as voltage fluctuates due to to continuous changing draw on the battery then the battery capacity will compensate for the lack of voltage provided by alternator.
i'd be looking at weight factors.
same goes with battery relocations as well.. its all about capcitance