I did quite a bit of experimenting with fiber filler loads some time back.
In light .38 Special loads (or target wadcutter loads in longer .357 cases), with fast-burning powders like Bullseye, Unique, Green Dot, and HP38, it makes no discernable difference with accuracy, point of impact, velcoity, or consistency. About pressure I can't say, but there are no signs, as there shouldn't be for loads this light. My guess is these powders ignite easily and burn completely no matter where they are in the case
In mid-power .357 loads, about 1100-1200 f/s with 158gr Cast or Swaged Lead SWC and H110 powder, accuracy stayed about the same, but velocity went up just a bit and consistency (mean deviation) improved. But the point wasn't to increase velocity, and when the charge was dropped the accuracy and consistency decreased too. All of which was pretty minor anyhow, so it simply wasn't worth the bother. (Again, in loads like this that are way under the pressure maxes, there were no signs of increased pressure.)
In .44 Mag, however, using mid-power loads, 240 gr. Cast or Lead SWC bullets, and H110 powder, a little pinch of Dacron fiberfill made a noticable (but not drastic) difference in velocity and mean deviation. But accuracy was about the same, and again velocity wasn't the goal, so eventually I abandoned fiberfill for .44s as simply not worth the bother.