Depends on why you're asking. If your cat is OK, it's unlikely you'll see additional power, just additional noise. If your cat is plugged, take it off, smash out the guts and reinstall it empty. I know that this seems anti-environmental, but get this: VW delivered lots of Vanagons with empty catalytic converters. Seems that they had designed the U.S. exhaust to comply with California emissions specs. which required the cat. The rest of the country didn't need the cat to comply (something to do with classification as a light truck). Rather than redesign the exhaust and have two systems, VW just installed empty cats on the Fed spec. Vanagons. Ours was one of those. No sign of having been reamed out. No guts were ever in there! If you get emission inspected where you are, just be sure that your engine is in good tune. The cat could make a poorly running engine look clean to an exhaust gas analyser.