Agree to a certain point but cats on post 2001 motors with 2 o2 sensors on EOBD with EOBD cat monitors I do not budge on, I tell the customer that yes he can have a cheaper cat but the fault will re-appear which it invariably does a few weeks down the line and I will not bear the cost of sorting it out, it is Genuine or he goes elsewhere for someone else to deal with the excuses when the cheap replacement fails and back comes PO420. The worst culprit I have come across for this problem is Foci following a coilpack misfire