A Boxster S muffler can be bolted right onto a stock 2.5 car's post-cat pipes, if that helps answer. Those pipes are the same diameter. I just did this swap because my 2.5 muffler hanger brackets were broken and a friend donated his old S muffler that was lying around. I did have to get the Porsche mounting adapter kit to hang it, though -- this kit is labeled for use mounting the PSE on a 2.5, but the reason is the same, to mate the S or PSE style muffler to the transmission hanger bolt locations. However, the pipes are a direct match.
Ah, my pictures and adapter part number at this link:
Used Boxster S muffler with mounting adapter by
bombaydigital, on Flickr
Threaded Mode
