Hi M Julian, my cat died a couple of years ago in her 21st year from kidney failure. When she was 18 she had an operation on her thyroid and she was absolutely fine, up and about almost straight away. I'm so glad I had it done, it made such a difference to her quality of life. I've had a ruptured cruciate ligament myself from a skiing accident and it's darned painful for a long time so I sympathise with your poor cat. My dog has had a heart murmur for most of his life and he charges about mad as a hatter even though he's getting on a bit and it has been no trouble to him at all. If the vet hadn't diagnosed it we would have no reason to suspect there was anything wrong with his heart.
If I were you, and your vet does at some point recommend your cat for the operation, as long as she is not showing any deterioration because of the heart condition, I would go ahead and have it done.
Good luck to you and your cat, whatever you decide.