It's based on the Lamborghini Veneno. Imagine some minor change in appearance that changes it just enough so it wouldn't provoke copyright issues from Lamborghini.
Nobody really knows what "SA" stands for, but according to unconfirmed rumors, the car was developed after the CEO of Rotor Industries had demanded a "somewhat quickish vehicle for a sunday afternoon cruise".
Being the highest-end sports car in the game, it's unarmed, so the mass of weapons wouldn't slow it down. Also, because if it would also be armed then nobody would ever drive anything else and it wouldn't be good if the game would have a 1 best car and a bunch of inferior others.
- I wanted to add a Zentorno-equivalent, but looking for pictures, I found a much epicer car. Also, the Zentorno is actually composed of parts of 3 real cars.