I would definately recommend a well prepared raw diet. I've seen it do miracles ro cats with gastrointestinal "issues". I know of one cat who was condemned to death by every vet because of her IBD. No medical diet worked and they really tried every single one availabe on the market. The owner was quite desperate when she asked me for help. We first started out on a cooked diet since she had nothing worth the name immune system left and she did better on that diet, but she didn't become really healthy so we (me, the owner and a vet) decided that we should try a raw diet... Within a week on a raw diet the cat was back in business!
Today (after appr. 4 month) she's perfectly healthy and the vets that treated her consider it being a miracle.
I have friend of mine who had a cat with basically the same problem as the cat mentioned above but he wasn't quite as ill as the other cat and this cat made a full recovery on a home cooked diet. We started out with cooked food there to of the same reason, he had a very poor immune system and we didn't wanna take any chances. He's completely healthy today and the owner have switched to raw and the problems are gone.
So I'd recommend starting with a cooked diet. If that works out well there's no need to feed raw (if you don't have any other reasons for feeding raw that is).
Take a look at:
www.littlebigcat.comwww.catnutrition.org
You'll find recipes on both raw and cooked feline diets there and lots of info. Start searching the web and reading books about it. make sure you have enough knowledge to make your own catfood before starting.
Before even considering raw, make sure the meat is safe! I only use Swedish meat (free from antiobiotics, hormones, parasites and always checked fot bacteria such as salmonella before it reached the market) and mostly organic meat.
. I hope that Bella will be able to make an improvement like the other cats did. 
