I don't want to worry you unnecessarily (but you asked!), but the one thing that comes immediately to mind based on what you wrote, is possible Feline Hyperthyroidism. Where the body kind of gets "revved up", and increased appetite, increased activity, and decrease in weight for elderly cats, are the symptoms. Hopefully the vet will check for that. If that happens to be what it is, it is treatable. I had a cat with that years ago, and got it cured by a moderately expensive procedure (radio iodine therapy). There's tons of other stuff it could be too, don't get too freaked out, but it's something to bring up as a concern to the vet perhaps, and hopefully you can get some answers.
http://www.catinfo.org/?link=felinehype ... hyroidism_Let us know how it goes.