-
Depends on the car
Well more what you consider an upgrade.
Having had phat installs before, I think a simple speaker upgrade using the stock sizes paired with an amp, and maybe a bit of matting in a couple of places is sufficient. That is usually not hard or expensive.
Yes if you want a sub you've got to house it. But that's the same in any car.
Obviously a proper sub and full pro install taking into account the car type, acoustics, and carefully paired and balanced equipment will deliver the best results. But that's usually OTT for the average drive.
Depends on the car. Not many places you can physically put a powered, enclosed subwoofer in a 996