Yeah the cat back is resonated... what I meant is that Piper also sell de-cats with a resonator in them to quieten it down http://www.piperexhausts.co.uk/piperstore/product_info.php?cPath=26_236_453&products_id=3547

I reckon you could just fit the new cat back without spending any more on de-cats etc, and then it'll be easy to just re-fit the standard cat back if for some reason you decide you don't like it.