When I had my Z28, allot of people used the magnaflow cats, no one really commented on how good they were but it was noted that they did cost power. I don't remember but it was in the neighborhood of 10-15 rwhp. Most guys used the RT cats with about a 5rwhp loss. The new Metal Matrix RT's don't seem to have a loss at the wheels. I do know the the Mac's, Carsound and the Magnaflows didn't work well on the F-cars compared to the RT's.