There have been some analysis(es?) of kitty litter, here is one http://www.thekrib.com/Plants/Fertil...er-boingy.html
The thing to keep in mind is that -- as you say -- each brand is completely different (not made for aquatic use!) so the scale is big, you could end up with a stinking mess that kills every living being in its proximity, or you could get a very cheap substrate that grows plants better than fluorite.
Same for clouding, effect on water chemistry, nutrient exchange capacity, etc etc.
I am using a brand called Special Kitty in my tanks, it looks like flourite once it is rinsed (just green/gray instead of red), does not cloud the water, grows plants great, and does not effect the water parameters. Actually I changed from this Kitty litter to 100% Flourite in my 10 gal tank and experienced major algae problems, and the Glosso that was doing well before has ceased to grow completely. Could all be coincidence, of course.
Now this is true only for that particular bag that I bought! No doubt that the bag you buy will be different, raise pH to 10.5, create a milky haze that never goes away, and kills all ghost shrimps. This risk is the price you pay for being cheap.