An exhaust upgrade has been dyno proven to have results, not to mention a better sound. Heck, even Nissan technically offers an aftermarket exhaust with the Nismo S-Tune. Most high flow cats are technically "off-road" use only, so your mechanic is technically right about the not being legal part. Most don't usually throw a CEL though, unlike some test pipes. You gain horsepower by removing the restriction created by the cats. Unfortunately, it also has the counter-effect of being less emission friendly. I have no idea what he means by 4 cats. Chances are, your car will be fine. An exhaust upgrade is one of the safest mods you could do. The worst thing that could happen is you mess up an 02 sensor, but its unlikely with HFC's.