Jeff, my biggest problem with bumping a stock STi into ST or SP, is that its not a limited production car. Subaru produces and sells as many as they can and will continue to do so in the future. You have to look beyond its name. It is not a mildly modified WRX as it does not share major components with the WRX. The motor, transmission, differentials, brakes, suspension and wheels are all
very different and unique. You have to consider it a different car all together, rather than a mildly modified WRX.
There are several special breeds of STi that would qualify for an ST or SP bump, but none of them make it to the US shores. Subaru makes an STi RA (racing application) RA spec C (racing and competition) and S204. They come stripped down with no radio, no A/C, no ABS, no airbags, even bigger wheels (the spec C actually comes with steel wheels since Subaru knows the owner will be fitting the car with thier choice of custom wheel), better aero, ECU tweeks, titanium exhuast, etc. They are limited production and mildly modified. They are a perfect candidate for a bump over in class.
There are other cars that could be debated. For insance, the Z06. Its not limited production, but it is faster than a stock 'vette and factory tuned. The base model is already in SSU, so should the Z06 be bumped to ST? Is it fair to have them in the same class?