The accelerator curve is really cool. A lot of modern cars just have a sensor that detects your pedal position and a simple algorithm decides how much power to translate that into. It's like adjusting the mouse speed on a computer. Feels like you're driving a different car.

Having said that, the default curve is often the best curve. They put a lot more effort into getting it right than you would.

Kinda depends on the car. Volkswagen cars are pretty "hackable" with OBDeleven which is a wireless interface for the hilariously named "VAGCOM" protocol.

Almost every car company does something similar and has as long as they've had on board computers.

VW/Audi/Porche are all the same company and generally share the same electronics. A lot of gauges and features are considered "premium" so they just disable them for VW branded vehicles. There's also regional feature lockouts; IIRC North American VW's can't have their fog-lights and headlights on at the same time but you can enable it through VAGCOM.