An object will fly heavy end forward. So, for the sake of argument, let's say the nock end of your arrow is heavier than the point end. When you shot it, the arrow would flip over and fly nock end first. So, if your FOC is too low, the fletching is having to work harder at stabilizing the arrow so it doesn't flip over, make sense?

Switching from 100 to 125 gr. heads will slow you down about....3 fps, but will increase your KE by, oh, maybe 1 or 2 foot pounds. If your FOC is good, between 10 and 14%, and your groups are good, I wouldn't mess w/ it. Oh, it will also change the spine of your arrow somewhat too.