By downpipe do you mean high flow cats? The 370Z doesn't really have a downpipe since its not a turbo engine. There are exhaust manifolds/headers, cats/test pipes, y-pipes, and mufflers. Or cat-back exhausts which replace the muffler and y-pipe.
Berk Technology has high flow cats and test pipes out for the 370Z that had some pretty good gains on my car vs. the stock cats. Those are the only ones I have personal experience with.
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