Greyfox, you're obviously a self-styled web guru, and have yet to deny differences between new and fireformed brass cases..
It's as though you've noticed differences, or that many others have, and just don't get that our situations are suppose to be exactly the same.
OP is in a position to answer his own question, given that he will be fireforming brass anyway. And unless he plans to shoot only new brass, will also be shooting fireformed brass.
With 5 pieces of it, and any load, he can see if it will make a difference for him.
If it does appear to, then all is not a catastrophe either.
After all, with 100 new cases, he could roughly determine best seating depth and shoot a couple ladders with different powder.
And with all brass fireformed, sized, & as consistent as it's going to get, he can focus on the potential derived from his ladders. Shoot groups with a narrowed incremental load development & final tweak of seating.
But that's way out there,, having never been rationally thought of.
Sheet, we all mix new with old brass and just bang away at 1/3moa with it, right?