Like i once did in the wiki i can confirm you that Sherry does indeed boost silvers, bronzes and irons. She was my second plat ever so i was very very happy about her and had the enthusiasm to test it out. Did field her with silvers, bronzes and irons and they all got the 5% boost. I did this right shortly after when AWing got implemented for princess units. Back then my team not only had many golds but even still some silvers (like elaine and leannee as i where lacking some essential gold units with all the endless breaks and 0 gold rushes). She was a breaking point for my team