To get back to the original question at hand, is he worth the 2016 option, IMO there's not much doubt about that now.
We MAY have been able to use the $13M better elsewhere, or that $13M may have been used to pay the salary of someone who doesn't have 8 wins and has pitched well out of the pen, too. One never knows. What we do know is that Clay has pitched like the Clay of old recently, albeit only for a part of a season - but OTOH we knew that's what we were getting when we signed him for $13M - a part-of-the-season pitcher. His finding himself when he did has made him worth the $13M.