Yes it is 2.5 inches, and also, look around before you buy them. The guy who put on my X offered to weld in a couple of them for me at only like 40 a piece.
I was considering on doing this except the way I was thinking was adding flanges on the pipe after he cut it up then add flanges to the pipes he just cut off, get 2 cats weld on some piping and flanges to those and this way you can swap the cats off the pipe with straight pipes a lot easier then having to remove the entire mid pipe every time inspection rolls around, just an idea I was tossing around.
Ya I know what you mean but I want my car to be loud. And I know i will attract cops and I dont want them to pull me over for loud exhaust and look under the car for cats know what I mean. Its a peace of mind thing.