i think the v6 ones might be a little shorter, cuz i know the cats on the v6's are longer then the ones on the i4's. you could always call the company that makes the cat back youre looking at....im guessin apexi since you said WS2, and ask them if theres any difference, and if they say no, ask about the length of the midpipe for the v6 and for the i4. if theyre not different, get whichever you would like. but yea, thats the only other thing i can think of other then the y-split section of pipe.