A common response to the problem is to extend the trait ladder with additional adjectives. Alas, this complicates matters, and makes it more difficult to memorize and internalize the trait ladder. One of my solutions was to arrange the extended traits in alphabetical order. Even with this fix the extended trait ladder is cumbersome and one is forced to resort to adjectives that are really only synonyms for Superb and Terrible. One of my other solutions was to add one "Very" for each level above Superb or below Terrible, e.g. "Very Terrible" or "Very Very Superb." This, of course, can quickly become ridiculous and distracting in itself.
Then it struck me like a song by Madness. What if we merely translated each extended level as "a step beyond"? Instead of Terrible −1, the character performed One Step Beyond Terrible. Instead of Superb +2, the character's achievement was Two Steps Beyond Superb. This is something characters themselves could pass off as a figure of speech, but which is directly meaningful in game terms and very easy to remember. Technically, the math is still there, but it sounds like something a person could say outside of a role-playing game. "That stunt wasn't just superb. It was three steps beyond superb!"
Four Steps Beyond Superb
Three Steps Beyond Superb
Two Steps Beyond Superb
One Step Beyond Superb
Superb
Great
Good
Fair
Mediocre
Poor
Terrible
One Step Beyond Terrible
Two Steps Beyond Terrible
Three Steps Beyond Terrible
Four Steps Beyond Terrible
Maybe it's a stretch, but I think it's crazy enough that it just might work.