Performance gains naturally aspirated with a cat-back are going to be somewhere on the order of negligible. It'll look cool and it will be louder, but it's not going to gain you anything appreciable 0-60 or in the quarter.
Now if you want to do a full NA build... intake, cams, P&P the heads, headers, full exhaust and a premium tune you stand to pick up a respectable amount... but that is going to be sinking a lot of money into a car that you "plan" to turbo later.
If you wanted speed you should have saved for another year or two and bought a WRX. Heck, you could have had a used one for the price you paid on that new 2.5i. The base Impreza may be fun, but it isn't a fast car and trying to make it fast is going to eat up a lot of money.