There are MAJOR problems with "Kitty litter." Number one, it disolves in water, eventually, it will disentigrate and foul your water. Also, it is very hard to get a "Kitty litter" with no perfumes or scents or other chemical treatments to disguiss oders (
most people don't like the smell of cat poop
).
Then, if none of that was a problem, in other words it was inert, and not water solluable, and didn't foul your water, it STILL doesn't do anything. Think about it, it is designed to absorb stuff and hold moisture. In a dry pot, it would be ok, and that may be where the idea of this whole thing started. But in an environment completly under water, moisture retention isn't that important.
The only possible benefit that ANY substrate can have is mineral/nutrient supply. That is why certain clays are beneficial (Iron supply). Now, Iron turns stuff red and I have yet to see a red "Kitty litter."
So the answer is: Not only is "Kitty litter" useless, it is actually harmful. I would spend the $5 a square foot for add-in "Laterite" and mix with a gravel of my choice.
BTW, I went with something a little more elaberate on my soon-to-be-set-up 55 gallon tank, but not everyone wants to spend $60-100 on substrate. (I found a good deal, and I splurged)
Good luck and Good fishing!