My Corsa 1.5TD appeared a little more sluggish than it should last summer so I took the system off, openig up the box from the top, and removed the brick before welding it up again. You can't see it's been done and it certainly seems to go a lot better now.
This week I took it for an MOT and was concerned about a possible emissions failure. Not only did it pass but the emissions figure was low at around 0.50. It suggests that the cat is actualy harmful as it will inevitable require more fuel to obtain the same performance with it still in. The car has done over 100k miles too.