There is not much to be gained when lightening a stock flywheel. 

It is not how much weight is removed, but where the material is removed.  A few ounces could be removed on the vertical edge of the shell and hub or a few ounces on the outside of the shell.  Material removed on the outside of the shell will give a greater reduction in the moment of inertia than removing material from the vertical edge of the shell and hub.

I have machined a lot if flywheels but I do not recommend it.   There is not really a safe amount to remove.  How much can be removed depends upon the highest RPM the engine may ever attain.  The porting and pipe determines the RPM where the engine will spend most of its time but we cannot predict how high the engine is going to rev with a missed shift.  We cannot predict what RPM the engine may experience when mistakenly is down shifted rather that up shifted to the next gear. 

I have seen the outer shell of flywheels explode on machined flywheels but never on a stock flywheel.  The question was.......was the flywheel machined too thin or would a stock flywheel have exploded if the engine experienced the about the same RPM?