Mike,

Here is the SCHRS measurement formula: http://www.schrs.com/measurements.php#mainsail

Two sails built differently but with the same dimensions: if this is the case, their are still usually small differences in dimension, and if not, my opinion is the newer sail will perform better than the old sail, simply because it is newer. The cut and material impact this some, but as an example, different F18 sail (totally different lofts, totally different materials) are built to the same dimension, and yes there have been some advancements over the years but its always the best sailors at the top of the fleet.