Downpipe Question?
 

I'm getting in my S5 TII engine today. It's going into an S4 N/A. Not to be a complete noob, but do I have to get a downipe to work the exhaust? also, If i were to get an aftermarket downpipe (which would be ideal) will I still pass emissions? Does the pipe replace one of the cats in the N/A models? any info and downpipe offers would be GREATLY appreciated.