RE: 100 grain vs 125 grain
GENERALLY, I find that every 5gn lighter you go, you' ll gain ABOUT 1 fps. Thus dropping 25gn will get you 5 fps.
In all reality, you won' t notice much differance....SLIGHTLY flatter shooting, but nothing HUGE.
You will likely loose a LITTLE KE, but the reduced friction caused by the increased speed will pretty much balance it out to where you' ll have about the same penitration.
Reasons for going to a 100 gn head from a 125? If you have too much FOC (Front of Center balance) you might be better off with a 100 gn head. This could be the case if shooting feathers, and light carbons.
If your grouping well with the 125s, leave well enough alone.