If there is a more free flowing exhaust, there may be a possible gain. Less back pressure is always desirable, with a turbocharged engine. The amount of gain is dependent upon too many variables to generalize it. In most cases, the gain won't be realized unless the transition is drastic (OEM to monster size, and a bunch more fuel). It depends on your current configuration, and the limit of your intended modifications. In the end, the ROI (Return On Investment) will still leave you with a net loss. If economy is your goal, look elsewhere.