good luck with the op- apart from the risk of damaging the parathyroids, which I'm sure you've been told about by the vet, there is little downside to it, especially at her age
some cats have dispersed excess thyroid tissue, which remains after the removal of the glands, and goes on causing problems, but this is more likely when no actually tumour can be felt in the thyroid - so I think you have to go with your vet on this one - if it doesn't work, or further problems occur down the line, you can deal with those when it happens