When you lease a car, you do not own it. The bank owns the car and depending on the country of residence, the dealership is possibly on the hook for a guaranteed buyout price. The lessee is certainly not allowed to "tune" the car without the consent of the bank/dealership. They are usually not even allowed to service it anywhere but at an authorized garage.

This is different from outright purchase of the car, where you can literally drive it to the nearest scrap yard and have it crushed right after purchase :)