I believe you can get both depending on the mod. Increasing efficiency will get you both mpg and power. For instance, underdrive pulleys. They remove some parasitic drag, which on the dyno adds about 5-7 hp usually. At the same time the engine doesn't have to work as hard, and mpg will increase slightly.
Another situation could be efficiency with the fuel mixture. The closer you are to a perfect peak fuel mixture, the better the power, and also the better the mpg. On my 350Z the factory had made the high rpm fuel mixture very rich as a safety margin to keep the cylinders cool, and last longer. The biggest thing Technosquare does as part of their computer upgrade is to correct the overly rich mixture, and pick up about 18 hp overall. The mileage also gets better by about 2 mpg.
One last thing lol. I swapped out the stock exhaust on my 350z with a true dual system. After resetting the computer my mpg (at 65mph) went from 29 to 33 average. HP gains for that system are claimed to be 12 hp. Not bad, and I gained power/economy by removing a restriction and being more efficient.
