It was a while back, 2003 to be exact that I repped for Pearson. I had a full line of their bows and 80% and 65% modules for each. Setting each of them for my draw length and 60# and noticed an average of 4 fps when switching from 80% to 65%. So it's not as much as you would think.

There are added advantages to shooting 65%, though. One is that it makes you use your back muscles more so bows seem to aim more solidly. Definitely get a cleaner release because of using back muscles and a better follow through.