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.
KE = m/2 x (SE / (m + mv))**1/2
where:
SE = stored energy
KE = kinetic energy
m = mass of arrow
mv = virtual mass of the bow
Clearly, KE is a function of the mass of the arrow. If we use an example of a bow storing 90 ft/lbs that throws a 350 grain arrow at 300 ft/sec which is 70 ft/lbs and we assume virtual mass to be 100 grains then that same bow will propel a 600 grain arrow at 240.5 ft/sec which is 77.1 ft/lbs. This is a 7.1 ft/lb or 10.2% increase in KE. In my opinion this is a very significant increase. In terms of momentum the change is from .47 to .64 slug ft/sec or a 37.4% increase. Certainly, super light arrows will travel alot faster but its also true there is a price to be paid.