Should I install drywall or plywood above a drop ceiling to increase the insulation value?
If your in a cold weather area a layer of poly creates a vapor barrier, otherwise Drywall and plywood have no insulation value. Notes for the insulation between roof trusses:

4" cavity space: R-13 or R-15 batts,
6" cavity space: R-19 or R-21 batts,
if it's a wide open attic, use an R-30 or R-38 batt.