Yes, it's basically just a straight pipe and it does replace the cat, but most people just put it on after, which is what I would do.
I'd have to disagree, bro. I have the kazuma on my celica and I installed it myself. If you wanted to take your catalytic converter out, you'd need more piping than what comes with the kazuma exhaust. The kazuma is designed to be installed right after your catalytic converter. If you plan on installing it your self, be prepared to cut your stock exhaust off just after the catalytic converter, you can't just unbolt everything and bolt the kazuma right up. The Kazuma will slide over the pipe you cut and you can either use muffler clamps, or weld a flange on the kazuma and right behind the catalytic converter, so you can take it off easily if you need to. (be prepared, with a Cold Air Intake
this exhaust is crazy loud) If you go the flange route, make sure you weld a flange on your stock exhaust, too, in case you ever need to put it back on.