I can shed a little light on this one. Excals stock arrows weigh 350 gr w/ 100 gr points.Using 75 gr points on them is not a good idea because you are going to be under their minimum weight requirements and it is really bad for your bow.

Jq7: Was that a typo on "carbon inserts"?? I am/was not aware of any inserts that are not either aluminum or brass.

Planter: you can come real close to figureing your speed by taking the advertised [and true] 305 fps and subtracting for changes. Example: going from FFF string to Excel = minus3-5 fps. Using a heavier arrow, subtract another 1 fps for every 5 gr increase in arrow weight. I'm shooting 20" LII's, 4" vanes, brass inserts, 100 gr points, weight 431 gr. w/ Excel string in my Phoenix. So, I've lost about 18-22 fps, which is what my "speed ring" on V-zone agrees with. Trust me, you'll never miss those fps and the benefits of those 431gr. arrows are numerous. As far as I know there is no spine issue w/ carbons. Hope this helps.