The only time your car will 'lose torque' is when you are launching. Once the car is in gear and moving, the lighter flywheel gives the engine less roatational mass to get turning, so therefore if anything you would accelerate faster. Although this isnt exactly true since the weight loss due to an aftermarket flywheel is very minimal compared to the mass of the rest of the car and all of the other moving parts, this is why you dont normally see lower 1/4 mile times after getting a light flyhwheel. So basically with a lighter flywheel you will have to rev higher when launching to get the same amount of rotational momentum to send to the wheels. The advantage to a light flywheel is how it allows your engine to change rpms a lot quicker.