Everyone agrees that the Mustang exhaust is restrictive, whether it's the exhaust manifold, catted h-pipe, or the catback exhaust. If you eliminate some of that restriction by replacing stock parts with performance parts, you can add horsepower. What happens, however, if you replace all three components? I installed Hooker LT headers, o/r x-pipe, and Magnaflow catbacks, hoping to increase my horsepower. I gained 23 rwhp with the headers and x-pipe. But, when I traded out the stock catbacks for Magnaflows, I actually lost low-end power. Using a G-Tech, my 0-60 mph time went from 5.1 seconds to 5.6 seconds. At higher rpms, I appear to have gained power. I found a thread on stangnet that explains the physics behind this loss in power: http://forums.stangnet.com/showthread.php?t=458814&highlight=catalytic+converter.