Your really gonna want to do a ton of research yourself.
this is all that I found after an hour and a half of searching the correct answer for you
.....someone put just a catback on and tried it and got 2 mpg improvement
....and others were talking about getting tuned for better mpg.
... keep in mind that our cars ecu's take a learning curve that may take some time, which means that if you change something, than your car will need to feel out the new part or fluid to run it efficiently.(if not tuned for it) although cat backs dont need a tune
I had even saw somewhere in my hunt that a guy had a super cheap GF who was running 87 in her wrx
...well, her ecu learned the 87 and got her 23 mpg, and when he talked her into upgrading to just 89 she only was gettin like 18 because the ecu was trying to keep from knock and didnt realize the improvement right away, because 89 was new to it and it had not learned it yet(though it was better). Even 93 would have taken time to learn after knowing 87 for so long. Thus tunning for 93 would be appropriate at this time.
I am only using this scenario so you can maybe take in the learning curve of our cars.
anyway, keep on looking at different companies info and search all the forums till your eyes hurt.