Lets look at this another way.

What is going to travel farther a ping bong ball travelling at 300fps or a golf ball at 250fps and which one is going to hit harder when it arrives. My money is on the golf ball

Another thing about a heavy arrow is it will absorb more of the bows energy thus making the bow a little quiter and have less hand shock.
A light arrow is not going to absorb all the bows energy and that energy has to go somewhere namely back through the limbs into the riser the end result more noise and hand shock.

Go over to the trad gang's web page and read the test results from Dr Ashby about penetration, FOC etc