I think I can answer this one, as you might know I own a STi and my cousin owns an MSP which I drive ocasionally and do all of the work for.......the main difference is in the drivetrain design and the weight. In case of a regular WRX, in the begining of 2002 one of the magazine tested a WRX vs MP3 at Laguna Seca, the MP3 won the race. In other words the extra weight and drag of the awd system took away from the handling characteristics of the WRX and the nimbleness and lightness of the MP3 made up for the lack of power. Now you add 30 more ponies in the MSP and you get the point.
Stock for stock on a straight away the WRX will win, unless you have a modified MSP, in this case the awd drag limits the acceleration of the WRX, again, the WRX will be in front on the start, but the MSP will reel it in within an 1/8th of a mile.
Hopefully this will answer some of your questions.