One thing to remember when figuring out the change in arrow speed vs. a change in arrow weight......The KE developed is a function of the bow more than a function of the arrow. In mathmatical terms, the KE developed is the integrated area under the draw force curve for the bow, minus the bow inefficencies (friction, noise, etc....).

Given a bow with the same draw length and weight, the KE developed for a super light arrow will be nearly the same for a super heavy arrow. The light arrow will just be a ton faster.
There are some small gains for using a heavy arrow as the bow efficency tends to go up which will yield you slightly more KE, but this figure is normally < 1ft-lb.