Okay the fastest recorded 1/4 mile impreza is Tony Rigolli in Australia who is running 2.2 that has been bored and stroked to 2.4 liters. However the car is purpose built.
The highest horsepower Impreza (or Subaru for that matter) that I have ever heard of is any one of the Jun tuned Imprezas in Japan that are pumping out over 600 hp at the crank. All of these use a 2.5 block.
Regardless of these facts, your engine will handle a little (not a lot) more boost than a 2.5 liter. And whether you turbo or supercharge the engine you will have a little more room to play with as far as ignition timing goes.
Now whichever forced induction method you should use depends largely on you. If you are looking for a slightly flatter overall torque band and don't mind a little lag down in the powerband, then perhaps you should try a turbo kit.
If you would like a car that is always turning some boost, has a little higher peak horsepower, but has a fairly inclined powerband, then a centrifugal supercharger kit, like the one I am almost finished with, might be the way you want to go.
Either way the car is going to be faster than most domestic v8s, 2.2 or not. As long as you have proper engine management.
If you have any questions about any of this, feel free to email me. I do have some working knowledge on the subject at this point.