For the street, you need the stock (typically 30 lb.) flywheel to help get you rolling. You could go with a lighter flywheel if you are racing the car as you will be using much higher rpm's off the line.
But, changing out the flywheel is a lot of trouble and time for no gain on the street IMO.
if you are racing yes, if not what they said X2; a flywheel stocks energy only to be released when its mated with the transmission; a lighter flywheel means that you will need to increase rpm in order to release the same energy the stock flywheel was doing at a lower rpm because of its weight; a lighter flywheel will make you creep into the redline like a Ferrari engine, but Ferrari's are also 1500 lbs lighter than our 4200 lbs beasts; remember torque is what makes our babies move forward; you don't want to kill that if you are street performance only; there are many ways to shed 100lbs and thus gain a 10th on the quarter mile; that might not be the best way to go; imo