I don't think so. Just got the reducer from JonB as part of the whole pack. Cats, Corsa cat-back. Definitely need the 2.5 to 3 reducer.
I just had my local Pronto Muffler guys do the welding. The fit was actually tight enough on the install that I could drive it to the muffler place and not even worry about throwing a code or having it loosen on the drive over - didn't even need clamps on the reducer for the drive over.