It's πλειονος.
Although I think that would require adding τοῦ before καλοῦ, giving μετὰ [τοῦ] καλοῦ πλείονος, but even then we would expect μετὰ πλείονος [τοῦ] καλοῦ -- all of which is more of a change than from πλειονος to παιωνος.
In further support of this, we do have "τοῦ καλοῦ πλείονος" used here (which is an Usener Edition). I have not been able to find both adjectives put together (but only καλός being made substantive by adding an article).