You can get momentum from speed, you just dont need as much speed to get the equal amount of momentum from a hevier arrow.
You are correct in that you don't need as much speed but that's not the point.
Model "X" bow set at "X" draw weight and"X" draw length will store a specific ammount of energy. The lighter the arrow the less time it spends attached to the string and therefore the bow is transferingless of that available energy stored in the bow.The arrow thenproduces less KE and less momentum. The heavier arrow stays attached to the string longer and more of that available stored energy is transfered to the arrow and this arrow has more KE as well as momentum and all other things being equal will penetrate deeper.
As for bibulls comment, it is true but the way he is talking it sounds like he thinks Im shooting a 200 grain arrow trying to shoot threw a moose length ways.
No, but if the hunter screws up and shootsthat moosesquare in the shoulder blade the 500+ grain arrow will penetrate quite a bit deeper than the 360 grain arrow will. Provided they are each properly spined and of the same diameter.