I would never alter my drawlength for a few fps or ft/lbs. What you gain is not worth the potential for the accuracy you may lose.
I got to ask this question so don't think it's personal. What is all this hype about KE? What are we hunting here? Elephants? 58ft/lbs is enough to take care of anything on this continent and probably most African game. If it's a matter of speed you want then just shoot a lighter, stiffer arrow. Here's a good site to log onto: Bowsite.com/bowsite/features/practicalbowhunter Look for the article on penetration and be enlightened.
Now to answer your original question. You should gain about 12 to 15fps with a 1" change in draw.