Well its seems I was extremely misinformed not by the forum members and was completey wrong on this topic i posted. I found out the 95 Golf GTI has a diameter on the cat to centre section is 53mm and the Magnex centre section is 55mm. So i need a reducer! Also the downpipe to cat bore size is 55mm if anyone is interested.
I measured the bore using a micometer thing my father in law let me borrow.