Hi Rod
Anyone please correct me if I'm wrong.
Lightened flywheels were used on racing cars mostly.
The reason for this is to make the engine get to top revs quickly, but you will suffer less Torque at lower revs.
Racing cars with large cams don't worry about this, I have a 192 in my car with a 40/80 cam so I need lots of rev's just to take off . If I drop the clutch at anything under 3,500 rev's the car will just die but it rev's out to 8,000 + . So if you have a stock or near stock street car I don't see much point in lightening the flywheel unless you are going to do club events or drags at the lights  
Another way guys with SU's used to get quick revs was to "cut" the dampner in the top of the carby ,but unless you want to get about 7-8 mpg this is not recomended  
Hope this will help you
Norm (nsw)