I agree its sad and how very tramatic that surgery would be! I can't even imagine! If the cat is showing signs of pain I think I'd lean towards doing it sooner rather then later.
It's a horrible thing to have to do.
We're having a struggle with our 15 year old dog
She has her good days and her bad. She still eats and dominates our younger dog but it's obvious it's getting harder for her. Sucky thing is she has no serious health issues other then old age. Sure she sufferes from things like loss of sight mild arthritis, loss of strength... but there is nothing really wrong with her that would make the decision easier.