It really depends on what you consider diminishing returns on increasing the weight of the arrow. There will be a point where there will be atrade offover speed which is probably different for each individual, getting to a standard will be impossible. Your arrow will absorb more of your bows energy as the weight of the arrow increases. However at a certain point the increase will be less and less until very little increases are made. Would that point be enough, if you add more weight, you can still get more technically. With the arrow materials being made right now though, I don't think there is a point of being too heavy. I am shooting arrows right now that are closing in at 700 grains and it would be difficult to add much more with what is being offered on the market.