It depends on the cam design.....and I hope someone with more experience than me can give us their input, but I'll take a shot.

On most cams the % let-off is determined by the ratio of the circumference of the outer lobe of the cam to the inner one. When you change modules on these types of cams you are actualy changing the draw force curve and storing more energy in the bow, and therefore will pick up about 8-10 fps. I've seen this personally on the Oneida Aeroforce, and Bowtech's Wheely.

On a cam like Bowtech's Infinity or Freedom they are essentially 80% let-off cams that achieve 65% by moving the draw stop to slightly shorten the draw to prevent drawing as far into the valley, thus stopping short at 65%. In this situation I have seen no gain in speed.