@mattiekane I see where your coming from, but; I thought the 2006 sti had a cat in the down pipe, then a small section between the centre and downpipe with another cat, then the resonated centre section then back box. All the centres ive seen are for 2001-2007, so replace the 2nd cat and deresonated centre. On 2001-2005 cars the 2nd cat and centre is one pipe.
My question is, is it safe to replace the 2nd cat and resonated section with a straight through section without having it mapped, as long as I leave the one in the downpipe?