One advantage of electric motors is that they provide strong pulling power even at low speeds. That allowed Ferrari engineers to concentrate on optimizing the V12's performance at higher speeds.
While the battery adds on additional pounds its size was selected to optimize performance without adding unnecessary weight. Having that additional battery serves a purpose beyond the additional power. According to Ferrari it helps lower the car's center of gravity for better cornering.
When a high performance luxury car isn't enough, buyers turn to tuner companies like Ruf, Hamman, and Gemballa to boost horsepower and change the car's appearance.