I would be more concerned about the weight loss. I really can't advise you. It depends how far you want to take testing, etc on a 16 year old cat.
Since the possibility of cancer has been raised, I think the "ghastly" procedures you mention are the only way to be sure. But even if cancer is found, the surgery to eradicate it is not something I would do with my old cat.
I'm not a vet, and I can only tell you what I would personally do, and I would not do anything that would cause prolonged pain or suffering in a cat that age. Many people would not agree with me, but as I said, this is just my opinion.
You can always get a second opinion before making up your mind. Good wishes to you and your kitty!! He's a lucky boy to have an owner so caring!