You have a classic air balance problem. The air is not going to the parts of the house at rates proportional to the house’s cooling loads. In other words, your downstairs is 40% of the load but getting 60% of the air. There’s no easy fix. You are far better off trying to increase airflow in areas that have low airflow…

True, to a point. The bottom line is that when you increase resistance in your distribution system (ducts), the air flow goes down. When the airflow goes down, cooling capacity goes down AND efficiency goes down (cooling delivered per unit of electricity consumed). You are essentially making your system smaller AND…

Sorry, that’s not a good idea. Location of return grilles has much less impact on air balance than location of supply registers. You want as much return air capacity as possible, regardless of where it is. Returns do not suck air towards them, the air is pushed toward the returns by the supplies. You are far better…