That's a great question and one I have no idea how to answer. I just figured more momentum is better up until the point that your arrow speed becomes unacceptable to you the shooter.

I won't accept an arrow slower than 245 - 250fps. Any slower than that and I start to get uncomfortable on longer shots (by longer I mean 25 - 30 yards).

Next, the question becomes: How heavy can my arrow bein order to travel at 245 - 250 fps?