Your friend is a moron plain and simple.

That is like saying I am going to buy a V6 instead of a V8 because I can make the V6 put out more power than the V8. Yeah you can, but only to a point and at a huge cost that is not worth it in the end. Start with the better product with a higher baseline then work from there, the cost is much less and the performance strain will be more tolerable to the engine and associated parts.

The Impreza/WRX is a fine car but the STi is the one you get for performance especially when you consider other items not directly related to the engine that are included in it.