From what I have heard is they are all 3" BUT all of them also drop down to 2.5 at the downpipe just like the Cobb.
that seems weird... i remember mounting my 3" spt a couple years back and it matched up perfectly with my 3" invidia dp. Unless I missed something... Bump for a very affordable, nice sounding exhaust!
Mine matched up fine to my Invidia dp also, but if you look under the car you can see it tapers 2-3 inches from the flange.