DON'T remove the foil(?) vapor barrier from the flexduct. It's essential to the integrity of the duct. If you must, you can run your existing (R8?) flexduct inside a larger sized flexduct or replace it with metal ductwork which gives you more flexibility with insulation (although it's still somewhat limited).

I take it your ductwork is in an unconditioned attic? A third option is to build a dedicated chase/soffit with additional insulation.