weight vs heel
If you are heeling too much, you will loose speed and this boat will typically develop a lot of weather helm and "round up" to windward. Adding weight, if it is low enough, can counteract the heeling. But, you can accomplish the same thing (reducing heel) with sail trim controls without adding weight (drop the traveler, reef...etc.). All things equal, a heavier object will move slower than a lighter one under equal forces. In anycase, you are going to be limited by your theoretical hull speed (unless you surf down a wave - see recent discussions on "broaching"). Play with your sail trim before using up your limited storage space with bags of water. That way, when the wind dies down, you can change your sail trim and move better than lugging around extra ballast.