I'll tell you what helped my cat is Hills' w/d food. I give her canned and
dry. Part of the time she gets regular cat food, but she gets at least a
half can of that every day and since we did that the inexplicable diahhrea
she had cleared right up. I mean to say I saw a difference within 24 hours.
I"m sure it is the high fiber content. Something to think about trying
rather than low residue.

Please consider trying a raw diet for your cat. My Bruno had loose
stools when I adopted him and the vets had me give him a special
Science Diet which did no good at all. And he didn't like it. I tried
giving him just raw meat to see if he would eat it. He got raw ground
turkey. He liked it. Then I got Feline Instincts powder and made up a
batch following their directions. He liked that. The diarrhea stopped.
His coat is silky and the stools don't stink up my apartment. He had
been on raw diet for a couple of years.

My other cat at some at first but he eats the dry food Wellness Super5
Mix and neither one have stool problems.

I know it is more expensive than commercial grade food but when I think
of the thousands I spent on vet bills for illness caused by diet:
Chronic Renal Failure, Inflammatory Bowel Disease the spending on
premium diet is not expensive. More importantly the cats are healthy.

You might want to check out the Yahoo group for feline raw diet. They
are a bit didactic but they have some good suggestion.
Also you might like to look at the Feline Instincts web site with a
picture of my Bruno.
http://www.felineinstincts.com/successStories/successStories-idx.html

Mary and the SF Kitties