They look reasonable but you really need the dual exhaust IMO. You could have a genuine one for that money too
http://www.ebay.co.uk/itm/Genuine-Jaguar-XK-XKR-XKRS-Rear-Diffuser-Areopack-Speedpack-Red-Colour-/252191511928?hash=item3ab7c91578:g:ffsAAOSwgyxWW3ob