I have a 4" cat back exhaust from RBP but would like to replace the down pipe from the turbo. Would it be good idea to change the down pipe and remove the cat? I live in Kaliforniaa so the cat part might be out of the question. any advice?

Is the flange from the 3.5" stock pipe to the 4" RBP kit welded on or a bolt on? If its a bolt on you can get an MBRP front-pipe and it will be a direct replacement. If its welded on, you will have to cut. As for the turbo direct pipe, check out Diamond Eye. They are a very well made piece, but a little tricky to install. Please let me know if youd like a quote on anything.

Put the down pipe and kill the cat.