If you put on hollow cats, make sure that you use MIL eliminators so that the rear O2 sensors don't set a check engine light (and richen the snot out of the A/F). MIL eliminators are easily made, or can be purchased for about fifteen bucks each. I used one, when I took out the cats on my Mazda Protege5. It worked like a charm, never set a fault code (and fuel economy stayed the same). Cooler yet, it netted me a few more horsepower in the process - a very noticeable difference in that little car.