not sure as to your tip issue, as my dealership does not sell them seperately, but what year is the S55 donor car? the NA motors (pre '03) do have a different exhaust than the Kompressor motors (which is what came to mind as I see way more of those than the NA models). i'm pretty sure a NA S55's cat-back should bolt right up with little or no modification, but check it out. caveat emptor or something to that effect.