Cats can be cleaned with citric acid. See here
http://bit.ly/13atVTX
For what it's worth, those reasonably priced magnaflow cats are high flow, aka about 1/2 the element inside of a normal cat. I was into the SVT mustang scene for about 6 years, some guys occasionally would throw a code with the high flow cats on there x/y pipes.
Edit: Wow, there is actually a thread a few down called "cleaning the cat" that is about using citric acid.