A car being a sleeper has nothing to do with it being slow, on the contrary in fact, a sleeper means a car which has great performance levels whilst it doesn't immediately appear that way.
So an understated or unassuming look combined with great performance, a wolf in sheep's clothing.
Then don't put any aero parts on it, leave the stock tires on and no carbon fiber hood. Then, mod it up with engine parts. It will look stock but be fast.
I found a lot of improvement from putting the Maserati on a diet. Do all weight reduction and leave the rest stock or close to it. With about 450 hp the engine goes and sounds great. Add a few suspension tweaks(have sports kit on mine) a close ratio(or race) transmission and mild drivetrain improvements as well, you then have a 520pp Maserati that drives and performs very well.