Say good bye to low-end torque with that Intake. I would strongly suggest retaining all of the stock torque reserves and intake tubing. Also, with an open, exposed cone filter like that, you may actually lose HP instead of gain it. This is because our engines generally run on the hot side, and open intakes will just suck hot air from the engine bay into the car's intake. In almost every test (that I have read) conducted by reputable sources, improved panel filters that retaint the stock boxes, or cone filters that are heat-shielded, produced the best HP gain. I personally replaced the stock air box with a shielded cone filter that draws directly from the fender wall, just like the stock filter did.
I have read some nasty reviews about that exact intake by the Impreza guys in the NA forum. You may want to do a search. Hope this helps.