Chris 249 said..
Define "effective". If you are racing hard with a good sail, a good mainsheet hand and there are lulls, flattening and easing the main can be effective. If it's full-on flogging much then it's bad for the sail and for speed, as others have noted. Getting the trim right is complicated and depends on a huge number of factors.
Mastheaders don't respond as well to fractionals to easing the main. They also can't use mast bend to depower the main in the same way. It also depends on the rest of the boat; a slender and efficient boat (like an Etchells) can sail with a full mainsail when a boat with stretchy sails and a telegraph pole for a mast must reef. The efficient boat has less drag so it can just be allowed to point higher in the gusts to depower; if you do that with an innefficient boat is just stops.
i agree with every thing said here by chris though our old girl is at her fastest when main is strapped down traveler is at the stop . My rule is that as long as the leach of the main is working and we are not stalling the keel and going sideways i dont need to change down headsails which works well on our tasman seabird though with ior boats (big headsails tiny main small fin ) this is often not the case and main needs to be reefed early or weather helm becomes uncontrollable
