Well I took the plunge on a dozen X Impacts. Will know in a few weeks' time if I made a mistake or not. Worst case scenario, I'm back to figuring out a hunting arrow set up and I just made a donation to my local archery shop.
The decision for me was less about the diameter and more about making my target arrow weight. A similar Rampage build was getting up there in weight and would have been below 270 FPS easily. I know speed isn't everything, but my thinking is a slightly faster arrow might help me out in situations where I think the animal just stepped parallel to me, but in reality it took a step away from me.
The time for analysis, seconding guessing, and messing around with calculators is over. Since the weather is improving, it's time to hit the 3-D range and see how these fly.