My understanding is that a certain stat helps govern how much a person can carry. Based on that understanding, a boost of that stat should increase carrying capacity. However, that does not happen.
Additionally, Infusion of Featherweight decreases the total amount of weight carried by three (my Total Item Weight goes from 185 to 182), but the character is still treated as though being at max capacity, and cannot pick up an additional 3 weight of items.
I don't know if this has already been reported because my search skills are lackluster.