I have never seen anyone add FI and not take a fuel economy hit. Whether you are using the power or not, you are still altering your fuel map. Then of course there is the extra hardware (plumbing on a turbo, pully on an SC) which introduces mechanical losses, and of course the extra weight. In most cases, the losses are worth it because you are going for performance. But for those who are looking to add power to a DD, think about what you are doing. Any time you start working with add-ons, you are introducing the possibility of problems, both initially and later on.
There is a reason they call it mod-hell
