Everybody I know who has fed their cat IAM's has ended up with a sick cat
.
The colon will produce mucous just like your sinuses do, however, it is normally such a small amount that it's not noticeable. When the colon gets inflammed, they will produce a lot more, once again like your sinuses when you get a cold. The blood may be caused from sores due to the inflammation.
Did your vet state IBS or IBD? I'm thinking it is more of an IBD (Inflammable Bowel Disease, rather than Irritable Bowel Syndrome).
Cats are made to eat
raw meat, not grains, not corn, not potatoes, period. They can not digest Carbs and therefore do not get any benefit from them, except to add filler to what they eat and to produce large feces that can fill up the colon. Undigested carbs can also irritate the colon, causing inflammation and the symptoms that you have described your cat is getting. Some cats won't be affected by carbs, others are.
I know of one cat in particular that will end up with diarrhea and blood dripping from his anus continually and it got worse as the cat aged. The owner now has him on a raw diet and it has cleared up and has perfect poops (it took about 6 months for the colon to heal). The vet also tried, their food, steroids but they didn't work.
I highly recommend this website for you, it is so full of great info:
http://www.catinfo.org/
Good luck with your kitty!!!!!!!!!!!!!!!!