ok removing the cats 4 bolts that come off easy right? now you got a section of piping thats open thats basically running off the header. what to do then is to take it to a muffler shop and ask them for a test pipe they will make a mid pipe with flanges so that you can remove the cat and swap it with the test pipe. as far as check engine lights go not sure.
Also this will not gain any hp. itll just sound a little more high pitched and the only differance youll notice is at high revs itll respond just a little better. again no real hp gains just a very small chance of a better response.
the only way i see horse power gains are possible is if your super charged or turbo charged. other then that i see no hp gains, and if there is some like in my case itll be at 2hp gain