Look at the piece of Y pipe immediately after they connect. You'll notice it's the same diameter as the two pipes that head into it. You'll also notice it's damn near 1" in diameter when it's crimped. You can't tell me that doesn't cause significan restriction!!!

Other manufacturers haven't fabbed them up, because Nissan was kind enough to use no less than 3 different cat. configurations/designs for the 1st gen Nissan, and there aren't enough of each to warrent someone making a production run of exhaust for them. So what they've done is market a slip-on as a cat-back, say it fits all years, and laugh all the way to the bank.