Any full length shaft weight increases spine,no matter how flexible it is.
It is because adding weight to the rear stiffens spine.The balance point is already forward of center so adding full length weight,puts a disproportionate amount behind the balance point,hence,a stiffer,dynamic spine.It can stiffen things a lot.
I've experimented with all manner of full length cords,powders and granular substances and all stiffened the spine.You will just have to try them to see.I found them all to be a pain.
Mark your balance point before adding a full length shaft weight.If that balance point shifts rearward after adding the full length shaft weight,that dynamic spine is now stiffer.It will not stay the same,regardless of how flexible a material you use.