I am curious as to what the heavier/lighter clutch weights do to the belt drive system in an automatic quad such as the Grizzly that improves performance? And are there downsides as well? Any info would be great, thanks.

 

You should check on some of the snowmobile forums to really get a goos idea of how these clutchs work.
Basically, heavier clutch weights will grab the belt harder, and will not allow the motor to rev as high or as quickly. Lighter clutch weights will do the opposite.
With a fairly low output, fourstroke motor, I wouldn't play around with the clutch weights, but instead concentrate on the driven clutch. You can change how the quad will upshift, and backshift by changing the helix and driven spring.