Think 600 to 800 grit wet. The higher number the better. However a fine polishing compound like tooth paste is often used to buff out plexiglass and other plastics. If the scuff is deep it won't buff out.

 

My wife's 2002 Red Monte SS had the same headlight problem. The car was being detailed last week. I threw the detailer a $20. He used a 4 step process with his high speed buffer after taping the car off. Polishing compound, wax, polishing compound and wax again. They look brand new, crystal clear and no scratches.