Clive, moving the mast step won't increase the forestay length. It will allow the top of the mast to be raked aft slightly,moving the center of effort aft, increasing weather helm, and possibly improving pointing in light to moderate conditions. It may also reduce headstay sag making, which will make it seem longer. However, if your headstay really is short, you will have to add a toggle where the headstay meets the stem head fitting. The only way to tell for sure is to put a tape measure on the headstay itself.
