Sorry if this is slightly off topic, but it does relate to the question of digestion.
First, are we talking about the same thing? "IBS" = irritable bowel syndrome. "IBD" = inflammatory bowel disease (e.g., Crohn's, ulcerative colitis). They are very different.
In humans, IBS is a junk diagnosis (diagnosis of exclusion), but it's primarily a motility disorder, not allergy related. Leaky gut syndrome is actually quite rare, and I have seen very little reliable (as opposed to highly speculative and logically flawed) research for it. Histopathology should be able to demonstrate it, so if it was as common as some people say it is, you'd think there'd be more evidence for it. Typically the folks who support the leaky gut syndrome in humans are also huge supporters of the notion of candidiasis, which also is much rarer than is often claimed (unless you have AIDS or a similar immune destroying illness). (I know for sure it occurs in celiac disease, which is an allergy to wheat gluten. Perhaps some dogs have a similar reaction to wheat.)
My lay understanding (and I have read a lot about this in trying to understand my own condition) is that most toxic build-ups are the result of localized ischemia, as one finds in, e.g., chronic myofascial pain syndrome. Again, in humans this is predominantly a psychoneuromuscular phenomenon, which may have impacts on psychoneuroimmunology but is not an allergic reaction.
Certainly if one is intolerant of a particular food, we see, e.g., diarrhea from irritation, which causes excessive and uncoordinated gut motility. Note that in humans, what one person handles well will send another into spasms of pain. This does not mean that we aren't evolved to eat certain foods; it just means there's enormous individual and subgroup variability. This doesn't mean that stuff is leaking, toxins are being built up, etc., etc. Of course dogs are more limited than we are in what they can eat since they are not true omnivores (at least not in the way we are).
I don't know how much, if any, of this is useful for understanding what goes on in dogs, but I just wanted to throw it out in case anything is of value.
P.S. One area where I think you could see a seriously problem with leaky gut (and here I'm just thinking logically, rather than based on evidence) is if you are feeding something that changes the natural pH of the gut. This is a huge problem in ruminants (cattle) who are fed huge amounts of corn, which they are not evolved to eat, and which creates an acidic gut (and the concomitant problems of constant bloat, infections, acid-resistant e. coli, etc.). Since I know nothing about the dog gut i can't say much more, except to guess that it would be similar in pH to ours. Do you know, Dana?