it gives the impression of a sealed box being ~10% larger tahn it really is. this extends low frequency response a bit. you should use 1 pound per cubic foot of enclosure volume
If your recommended enclosure size is say 1 ft^3, and you make an enclosure to meet that recommendation, would adding polyfill afterward not sound as good then? Or would it extend the low response without sacrificing anything else?
Is polyfill prettty much only used in cases where there is limited external space?
recommended size is just that. it's not an absolute. if the box is 1cu foot, add one pound of fill. see how it sounds. pull the fill apart so it's all loose and fluffy.