I have a problem in that I cannot leave well enough alone. I HAVE TO mess with my guns.
I did a bit of experimenting a couple of years ago with FFFg and cornmeal in a 44-40.
All of the following are volume measurements.
I loaded 12 rounds with a 200 gr bullet and 30 grs of FFFg, no filler.
I loaded 12 rounds with the same bullet and 28 grs of FFFg, 2 grs of cornmeal.
I loaded 12 rounds with the same bullet and 25 grs of FFFg, 5 grs of cornmeal.
I loaded 12 rounds with the same bullet and 22 grs of FFFg, 8 grs of cornmeal
I loaded 12 rounds with the same bullet and 20 grs of FFFg, 10 grs of cornmeal.
In all cases the fill level was visually the same.
I took these to the range with sandbags and an Uberti 1866 rifle.
I fired all rounds at paper at 30 yards off of the sand bags.
All group sizes were within the shooters ability to hold with Uberti furnished sights. Roughly as I remember, 2.5 inches
I saw no difference in POI.
I saw no difference in amount of visible smoke.
I saw no difference in the noise level.
I felt no difference in recoil.
Granted 30 yards is not a distance to prove accuracy or to establish a hunting zero but for the game we play it is certainly within reason.
The conclusion is: The filler in the above amounts does not affect the visual, physical, aural or olfactory experience.
The only possible benefit is in the economy. This in my opinion is a spurious argument in that in the lightest load you are saving .2¢ per load.
If you want to stretch the volume of powder then add a little filler on the top of your powder before you seat the bullet. No other argument will work.
My conclusion is that adding filler is a pain in the butt and certainly not worth the effort.