Frankly, you' re undervaluing the momentum you get with the heavier arrow. There is a sizeable difference. Going by the strict forumula and fiddling around with all those decimal points is confusing. You can take arrow weight multiply it by speed and then divide by 1000 to get a pretty basic index number that probably illustrates it better.

Take your 351 gn arrow at 282 fps. It gives you 98.98. Your 453 gn arrow at 249 gives you 112.797. So, round it off to keep things simple, you' ve got 113 vs 99. That' s a tad over 12% more momentum you' re giving up for a 13% increase in arrow speed. I don' t know how far you intend to take a hunting shot, but the difference in trajectory within 30 yards is practically negligible. But, you' re playing on your nickle so make yourself happy.