My 03 has a single 3" with a 3" cutout at the Y behind a blown LQ4. When I open the cutout, i have no change in boost, even bypassing 2 mufflers and 15' of pipe, lol. Same result when I ran open headers. I have to believe that it flows enough to not be the bottleneck. I am constantly getting comments on the sound as well, the bark is deffinately unique, LT's, resonators instead of cats, 18" magnaflow and a corsa sport.

Always heard that it's a toss up between si 3 or du 2.5 because even though the du 2.5 has more area for flow, it has almost twice as much surface area on the tube wall and therefor has much more friction loss. If you really want to open it up, do single 3.5" all the way out, that would be my choice.

 

im still running single 3" too. dont plan on changing it any time soon either.