Do cat back exhuasts increase performance?

It depends on the application and proper tubing sizing. A properly sized exhaust will net some form of gain, but don't pay much attention to the manufacturer's claims. The average performance gain off of cat-back only for an all-motor application is generally 4-7 horsepower and torque, at best. Turbocharged vehicles can net much more power and better response when upgraded from stock to a larger free-flowing cat-back. For practically ANY performance application, all-motor or otherwise, a cat-back upgrade is a MUST, though.