I have the Cobb on my STi, and also have the Perrin one piece sitting in my garage. I like that the Cobb has the bung for a wideband A/F sensor, but that can be added too. The Perrin unit stays 3" all the way, like synolimit was talking about, and I don't like that the Cobb tapers to 2.5" at the flange to the catback. Both of them were catted, and I am contemplating switching over my Perrin DP for the increase in size. The Perrin one has a separate pipe for the first few inches for the divorcing of the WG.