I have not come across any cases of catnip overdose where the cat did anything worse than vomit. Some cats know when they've had enough, and some cats don't. Luckily, there doesn't seem to be any harmful side-effects. So the cats that don't know any better don't risk harming themselves. I can't say that I've seen any behavior indicating addiction either.
I would still avoid feeding it to your cat excessively, since the large amounts do cause them to vomit. It also causes diarrhea, which means at the very least, it causes a disruption in their digestive system.
I would definitely avoid giving catnip to pregnant cats, as it's known to be a uterine stimulant.
Here's a link to the American Veterinary Association's article on it. They mention a study on rats that showed they were more susceptible to seizures while on catnip. I would take it with a grain of salt, though, unless you find the study and they accounted for the dosage and differences in hearts/size.
