what are the major differences between the 2 clutches. i have both clutches side by side and they look the same. also, i notice the 250 pressure plate has lightening holes where the 500 those hole are not cut out...if shaving a few grams off the flywheel helps the motor rev easier, how would shaving a few grams off the clutch set up help?

Angular momentum is defined as:

(The moment of inertia) x  (the RPM)

Since the clutch rotates at approximately 1/3 the engine RPM on most Quad engines you would have to reduce the moment of inertia of the clutch assembly approximately 3 times the amount that you would have to reduce the moment of inertia of the crankshaft assembly to have the same effect. 

The moment of inertia (flywheel effect of a rotating object) is a mathematical way of comparing the flywheel effects of objects rotating about an axis.