The recommended mast track position should be suitable for all sail sizes that fit that particular board’s range. So for normal sailing in a given board’s range, no, you shouldn’t have to move the track. Obviously though as sails get smaller or larger the track will be more forward of more back. For example, the 93 liter Futura has a mast track range of 127-140 cm off the tail while the 133 liter Futura has a mast track range of 130-145 cm off the tail. So if your mast track is all the way forward on both boards then the difference is actually 5 cm. Of course the sail ranges for both boards is quite different.
As for what setting is the fastest? That is easy and has already been answered. It is the setting that you are most comfortable with because in the end you are faster when you are confident.