I think any attempt to make a STi or a WRX into a sleeper is going to be futile. Face it these are performance cars. How many people sees an impreza and says, "that's gotta be an RS?" Anyone who is not already familiar, but knows the shape, with a Subaru is going to assume it's a WRX. And if they don't already know the shape, then they don't know what is a STi or a WRX or even a RS. Those people probably don't care about racing with you either. They will simply assume your STi or WRX is a "weirdly" modified Civic. If you are trying to fool a Subaru guru . . . then forget about it. No matter what you do, we are going to know what it is.
Even if you can get people to think that your STi is a WRX . . . so what? A WRX is already a pretty formidable car . . .
So really, who are you gonna fool?
Camera Hacker - http://www.CameraHacker.com/