honestly I did not noticed any change in fuel consumption. however if there is higher consumption then it must be improvement in HP too. which might be possible since engine will breath easier since there is no more silencer in the way so it should bring few HP. People who have noticed worst gas mileage probable have also drove it harder to hear and enjoy new sound and that might caused higher consumption
Bottom line there is no cheaper way to get better sound than stock. There are better sounds and exhaust on the market but price is high. Also if you don't want very loud thing for daily drive then this is better than any other solution.