I've fitted new Cats on my 850csi. I cut the Cats open at the weld on one end and removed/knocked out the Cat insides leaving the Cat cans empty. I then welded (stainless) Magnaflow Freeflow/Race Cats (200 Cell) inside the empty cans, they are a snug fit. Then reweld the end back on. I have also replaced the centre resonator with straight through pipies with a crossover + opened up the back box's and gutted them. You would never know they'd been split open. Exhaust still looks standard. Passed the MOT.