Not quite. Headstay sag will result in increased draft in the sail. This helps provide the power that the boat needs. A saggy headstay actually pulls the mast forward. Think about the geometry. If I add sag to a line, the points at the end must come closer together, resulting in the top of the mast coming forward. Pointing ability results from the angle that the leading edge of the sail makes to the apparent wind. If the halyard tension is loose, the draft is further back, resulting in a higher pointing angle in light air. This is why many of the top J-24's have ugly scallops along the leading edge. As the breeze comes up and the sail stretches a bit, we tighten the luff of the sail via halyard tension. We also tighten the shrouds, tightening the headstay, and flattening the sail since the headstay sag is less. BTW, the large amount of sag I was referring to in my earlier post is approximately 6 inches in the 24, and I am approximating between 8 to 10 is about right in the 30.
Light air - Deep Draft in Genoa, max headstay sag, loose genoa halyard tension
Medium air - Less draft in Genoa via tighter headstay and less headstay sag, start to tighten halyard. Don't forget that headstay tension also is a result of main sheet tension. As the breeze comes up and you tighten the main leech, you are also tightening the headstay.
Heavy air, tighten shrouds if not done already, tighter headstay, tighter halyard tension.
