Don Stagg of UK Sailmakers in Miami has been working with me and others on performance issues. Among his cogent advice was to add a second set of blocks at the boom vang to double the purchase and pull down the sail flatter. My typical speeds of 4 knots suddenly went to 5, a 25% increase. There is less weather helm and less tension on the mainsheet. If your sail is a little puffy from use, try this as an inexpensive and instant improvement.