Considering that the custom exhaust is most likely a 1-off they build directly on the car, it is impossible for them or anyone else to compare the performance of it to an off-the-shelf option because.........it doesn't exist until the make it for you.
As others have mentioned - in theory the off-the-shelf products will have integrated some R&D testing and time to refine the performance. In theory.
Most likely though, when we are talking about cat-back exhausts on an NA Z, you are splitting hairs on the performance end. They will all be pretty similar. I'd suggest getting the one that sounds the best to you... which will probably be difficult to predict if you choose the one that doesn't exist yet.
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