Race them, SQ, and SPL drag them
I say have the car be able to do either so that you aren't limited, but every car audio item should add weight and make it slower.
I think for the car audio we should have nearly everything. Different boxes, walls, subs with different specs (as opposed to only sizes), electrical upgrades, chassis stiffening (concrete floor, etc), dampening, different wire, processors, etc.
That could also really eat up some money in the game

