I have to agree with phil. IMO the heavier arrow "should" penetrate better. However like FL said...there are alot of factors involved. I've killed deer with complete pass throughs from bows ranging from 53lbs to 69lbs of KE. In fact, very few out of a few dozen have not been a pass through. So I also have to agree with Slo-bo's statement...
" In my opinion, in most cases, once you have reached a certain speed/KE, it becomes somewhat of a mute point, it's more a matter of how deep in the ground the arrow will stick on the other side of a pass-through shot",
with this addition..."with a sharp, quality, broadhead".

One theory I've always had, and whether or not it is any good(LOL), is that if you have adequate KE, on a shot into a live animal, the shaft size really doesn't mattter...you have a 1"+ broadhead making a hole through that animal...the shaft just follows the head through the hole...it doesn't really act like a target and slow down by friction. (Of course...if you dead center the shoulder then it would obviously be different than a behind the shoulder all flesh hit.) But you get my point.
Penetration...IMO is based on KE, but ALSO on broadhead performance...if you have a good head, that can drive through flesh and bone, (even on a bad hit), you will get good penetration. In fact, I think even if you have marginal KE with a really good head, you will probably equal or outperform a setup with alot of KE, and a poor head. I've been shooting Magnus 2 blade 125's and have had some incredible penetration out of my last couple bows, and none of them is a speed demon by any means(240 at max). I personally believe it is because of the head. I think alot of traditional archers realize the way these type of heads work, and that is why they use them. Even with low KE, you can still have great success. That is my humble opinion, and maybe I'm nuts...but it works for me <img src=icon_smile_big.gif border=0 align=middle>