Be careful if putting more insulation in attic space to not choke off the air flow within the attic. The soffit and ridge vents need to stay connected via a 1" airspace along the roof sheathing. You can used foam venting panels to maintain this air space and allow the insulation to be blown in around it.
Have you thought of encapsulating the duct system in rigid foam?
Have you thought of encapsulating the duct system in rigid foam?