will putting a bigger backbox on affect power to my sti 8 and will i use more fuel

 

Depending on backbox, it'll sound nicer, but I doubt that you'll use more fuel or gain anymore than 1 0r 2 horses.

For more power a full decat would be the order of the day.

 

In my WRX03 when I changed the Scoobysport backbox for a STI8 I lost from 7-10ps measured with DeltaDash road dyno, so YES a "good" muffler gives some remarkable power but bear in mind the rest of my exhaust is already decated.
JIM