KE and momentum are more important than speed. Almost always a heavier arrow shot out of the same bow will have more momentum and KE. Weight is more important. Think of it like this. What would you rather get hit with at 3 feet away, a ping pong ball or golf ball? The are the same size and about just as hard. Now throw both balls as hard as you can. The ping pong ball will leave your hand faster then the golf ball. But at 30 feet which will be going faster? The heavier ball will hold its speed longer (momentum). At 3 feet they both may sting the about the same but at 30 feet give me the ping pong ball.
It's the same with an arrow. A 350 grain arrow will loose it speed faster than a 500 grain arrow. At 20 yards the KE will be slightly more with the 500 grain but at at 40 yards the 500 will me much more. At 40 yards the 350 grain arrow will be much slower in comparision. It will lose a greater percentage of speed.
Guys have looked at this alot. You should be able to find lots of info on it. Search the mathews forum. It is a great resouce and no bashing like AT. Shoot through a cronograph at 2 and 50 yards with both a light and heavy arrows. The light will lose much more speed.
Here is a link to a KE calulator. Play with some numbers. 580 grains -250 fps and 350 grains -300 fps
http://forums.mathewsinc.com/archery-4/ ... calculator
I shoot a 70# bow with a 584 grain arrow at about 250 fps. Thats on the heavy side (8.4 grains per pound) Most bow makers require 5 grains per pound. I think around 7 grains per pound is good.
Heavier arrows make your bow quieter, absorb more energy from the bow and hit harder. But yardage estimation is more critical. With todays rangefinders not a problem as I see it.
Light arrows are nice because they drop less. If you think its 25 yards and is 29 not that big of deal with a light faster arrow. With a heavy arrow it could mean 6 " low.
I hope this helps.