Duct Diameter Reduction
Well, as your takeoffs progressively "steal" air from the supply trunk, there's less volume (and therefore less velocity) per unit time. This could reduce air "throw" from the register so that air distribution throughout the room is compromised. Usually though, you have the opposite problem - too much velocity from undersized ducts and oversized equipment. If your current airflow/distribution is fine, I'd leave the trunk alone. You may need to reduce the branch diameters if you go from flex to hard pipe - the resistance to flow for flexduct is about 25% higher than metal ducts.