This is a problem with 4* as well. I like having 50% XP bonus for color matching instead of the 20% for uncostumed leveling. The bonus becomes more significant if 2* or higher level feeders are used. What this means in practice is that we have a choice. We can either restrict the costume to a diet of matching 1* and stand a decent chance that the skill can max before the XP, or we can mix in some 2* and burn feeders after maxing the XP. It’s also disappointing that feeding a Li Xiu to a Li Xiu costume was only worth 40% after the XP max. Even if we don’t get the same XP to skill percent ratio, we should be able to expect a 4* costume fed a mix of 1* and 2* to level up the skill with the last ascension.
A bonus for using a matching hero would also help overcome this discrepancy, even if it’s only 20%. I run a pair of TC 20s, so I do have spare 4* heroes, but the only duplicate costume I’ve gotten is Brienne. We don’t tend to get duplicate costumes anywhere near as often as duplicate heroes. Also, we’re leveling the hero in the costume. I do plan to level both Melendors with their costume. The first one is complete and I’ve started the second.
I don’t know how 5* works in practice because I don’t have any 5* costumes yet.