I was reading and had to chime in.

I am NOT an expert but I did just go through a very similar situation. This is what We found.
For thelast couple ofyears I have shot the Easton Axis 400 @26"409grnsout of a much slower bow.My broadheads(100grn.Stingers/Buzzcuts) and fieldtips always flewtrue with no adjustments.This year I bought a new bow ( 101st Airbourne) Set @ 60 lbs w/a 26" draw.A much faster setup.I shot some low end Gold Tip Expedition Hunter 5575sfor practice and was pleased .....But I liked the AXIS arrows.So I went to the shop(Little arm twisting needed) They ran the numbers andwe decided on the ST Axis 500s with 2" Predator vanesand I bought a dozen $$$$
This week I decided it was time to get the broadheads out and WOW. I was all over the place. So I went back,checked the bowtuning(good) paper tuned (bad).
Long story short.....The 500s were not the correct spine for Me and My setup. I shot the 400s with both fieldtip and broadhead(100 grn.)and my setup is now spot on.My Archery shop replaced the 500s w/the 400s at no cost.That's standing behind your business!!........WAY
So if I read right......(Would a 500 Easton Axis ST be a good choice with a 100 grain tip?) It might.....But I wouldshoot some 400s and 500s @ your shop b-4 spending the $$$.
Just my two pennies!
Be Safe
Bill