At approximate equal distances, two six-inch ducts are equal to one eight-inch duct. You have much less resistance to air flow to the downstairs ducts ( 2-8's and one 14). You do need more airflow downstairs for the heating, but you need more air upstairs for the cooling. In actuality, the (6) 8-inch ducts upstairs provide enough air for 1-1/2 tons of air conditioning by themselves without the downstairs ducts.
I suggest that a technician install a Trol-A-Temp or equivalent damper in the 14-inch duct for use as a zone control with a thermostat for the rooms fed by that run. The air flow should be more satisfactory.