I would suggest joining this group: http://pets.groups.yahoo.com/group/FIPCatSupport/
I would say a couple of them are "experts" on FIP. If you post your cat's blood results there (just ask your vet for a copy), they will help you interpret them. I will look out for your post!
My best suggestion would be to be sure you ASK QUESTIONS and DO RESEARCH (search FIP, as well as your cat's symptoms to see what else could cause those). You are already on the right path by coming here and trying to find out more. I would also recommend a second opinion. My experience is that once a vet has FIP in their mind, it's hard to change it. Find a reputable vet in your area and take your cat and his blood work there for a second peak. They may very well say FIP as well, but at least you gave it a shot. I wouldn't even be afraid to ask for a third opinion, if you still weren't comfortable with the diagnosis. I had 3 vets say FIP about Zoey - and I KNOW 2 of them based that information solely on the "diagnosis" of the first vet. Had they looked at her most recent blood work, FIP would not have been on the top of their list (for the record, I believe she had an severe infection that became harder to fight after she broke her tail, somehow - a broken tail that was MISSED on the first look at the x-rays because the vet was so set on FIP).
Without knowing what other symptoms your cat has and what his/her blood values are, I can't give you a whole lot more information at the moment about other possibilities. There are a lot of them, however, and posting some info about your cat would be helpful.
The one thing that strikes me is that your cat responded well to antibiotics. From my understanding, FIP should not respond to antibiotics, unless it is fighting a secondary infection (which could certainly be the case). FIP is caused by a virus, not a bacteria. Standard treatment for FIP is prednisone and antibiotic (antibiotic only because prednisone supresses the immune system). Be careful with prednisone - if your vet puts your cat on it, and he/she does not have FIP, but some other infection instead, prednisone could worsen the infection.
Please do not write your cat off yet. And certainly don't let your vet write him/her off either!
I hope this helps a bit! I know it is very hard to get such bad news and to feel there is no way you can help him/her. But try to stay positive and hopeful!