I bought that same exact y-pipe from that eBay seller about a month ago. Installed it about a week ago along with ERZ cats (still stock muffler). Fitment wasn't bad, but not as perfect as I would have liked. (Loose fitted, things were off about 1/2".) Everything bolted up and came together though. They did send the wrong gasket with the y-pipe, so I had to reuse the OEM one.
Since both were put on at the same time, I can't say for sure how the y-pipe alone changed things, but with them both, there's definitely a HP increase and the sound is a little louder, but a similar sounding to stock. On cold start, there seems to be a little rasp. Since it's relatively new, I don't know if that'll go away or not. Once it has warmed up, at idle, it sounds exactly the same as stock. WOT, though, it's definitely louder. I'm getting my car UpRev tuned tomorrow, so I should be able to get a dyno read of what these did for the car.
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