I would think a heavy arrow, with the same KE, would penetrate better than the lighter arrow, by virtue of mass weight. At the same energy levels it takes more resistance to stop a heavy projectile than a light one. At some point there are trade-offs. Shot from the same bow a lighter arrow may perform better than a heavy one, especially at lighter draw weight. Running the numbers there comes a point where an optimum arrow weight is realized. Many things need to be considered. Shaft weight, length, and broadhead weight, to arrive at a good compromise of total weight and FOC. For my setup a 375 gr arrow (100 gr head) seems to work best, giving a good blend of FOC, KE, speed, and trajectory. Not to mention shooting and grouping well. As others have stated a well tuned bow is highly important to good penetration. An arrow darting and dodging all over the place is not going to penetrate very well, IMO.

Phil.