I wonder if the uncertainty about whether filler is helpful or not, is related to whether or not we're talking rifle or pistol, and also depends on the primers' ability to fully ignite the powder that may not be right up against the flash-hole. One would have to do a whole slew of tests to see why.
I also believe that the filler substance uses up some of the energy as it is melted and/or combusted. A completely inert filler that doesn't extract much heat (energy) would then be ideal. That would explain why the really airy ones like Dacron work, because they're mostly air. Denser fillers will degrade performance.
I once tried tumbling some very dirty .45ACP cases in kitty litter - unused by kitty, or at least I thought it was unused! Although most of the cases came out cleaned, I noticed a couple of cases had what looked like sawdust caked inside, about 1 mm thick, on the bottom around the flash-hole. I loaded them anyway with the rest of the cartridges, using the lowest (starting) powder weight.
When I shot the suspect rounds, there was a very unusual "POP!", as if there was no powder at all, the bullets departed downrange but did not touch my 20-yd paper target. I concluded that my "kitty litter filler" had consumed a significant part of the energy that would have otherwise contributed to more bang.