here is what you do to get around the visual inspection in the case a cop would pull you over AND get under your car. first if your car is lowered, i doubt the'll take the time to jack it up and look under there but anyways.

go to the junk yard and get a random cat. then gut everything out of it. then run your test pipe THROUGH the empty cat. so when they look, they will see the cat there and can't tell the diff. all you have to do is cut the ends of the stock cat to the size of your piping and have it welded.

i was thinking about doing this but then i just bought a carsound 2.5" high flow cat. gonna install it when i put my turbo on though. do what you want, thats just an idea you could do.