*whispers* the Wreath and Crown stack.
Omg! Duuuuuude, no way! Okay, well, luckily, I did finally get one to +8 AND made it a costume. Now to make another non-costume one. It does seem as if every upgrade has the same chance of failing or succeeding. In fact, the one I just made for Chisami failed at +1, then succeeded at every upgrade after that. On a different account, I failed at + 2, and +3, but succeeded again, until +7. Is it maybe a 50/50 chance?