The whole reason that the Impreza RS isn't marketed as much and sold more is because they are/were still experimental. Remember, they were just experimenting with a bigger (Forester) engine in their smallest car. It turned out to be a hit, but the quantities that they make of the car overseas for the USA, is very limited.
To really get the car sold in the USA, they'd need to open a plant at which they'd actually make them over here. There are a lot of expenses and whatnot in making the cars overseas and shipping, customs, etc have a lot to do with it. Granted they can probably get cheaper labor elsewhere in the world, if you are selling enough units here in the states, it will pay off having a plant right here.
Also, when they do market the Impreza, they market it as a rally car. How many people do you know that drive down dirt/gravel/forest paths & roads every single day in their _________ (fill in the blank) car and want a small car? Perceiving the car as a rally-type of car makes the world believe that it is only good on dirt & gravel. Personally I think it is excellent on tarmac, though not in a straight line.
With a better suspension setup, different tires, and some sort of variable valve timing and ignition system, this car would scream as a street car for straight line purposes as well. Possibly the engine needs to be built for some slightly higher RPM's and power up top, though the torque needs to stay about where it is at down low as well.
Damn, I just keep writing full articles today... sorry about the length.