An aftermarket downpipe wether catless or catted with throw a code stockLate reply. . . The answer is it depends... If you're hoping to see any gains, you definitely need to get it tuned. If you just want to bolt it up for the sound or cool factor, then I think it'll work and won't throw any codes (if you get the catted pipe), but it's not likely to gain you anything.
P.S. I recognize I'm late to the game, but maybe my reply will help someone else down the line.