2000z28ss

Hey, I have a diablo sport predator programmer at the moment, I was just curious if I should get my car professionally tuned to get the most out of the bolt on's I currently have, I only have the stock Diablo tune or the "91" octane tune, I'm getting REALLY bad gas mileage, 7-9 city, 11-13 highway, it doesn't make sense, i replaced all the things that have to do with fuel delivery. lines, pump, filter, etc, I was wondering if a professional tune would get me more HP, and better gas mileage, i don't really care about the mileage, but more HP is always better, if i can get both, that'd be freaking amazing. Any help would be appreciated.